Show helpful message or be silent (but do not fatal!) when configuration files have different source language codes and cannot be translated

Problem/Motivation

Follow-up to #2584603-48: PHP exception on manage fields after enabling Configuration Translation

we cannot actually prevent having config files associated to the same mapper in different languages (which prevents translation). If we only add the file to the mapper if the langcodes match we have a sort of "silent failing" in that (in this case) you cannot translate the node type label (because the respective file is not added to the mapper) but you get absolutely no feedback that something is wrong or why it is. Bad.

Step to reproduce:
1- Install a drupal 8 in Spanish.
2- In "admin/config/regional/language/add" add Inglés language.
3- Go to "/en/admin/modules" install "Configuration Translation" and "Content Translation" modules.
4- Go to "/en/admin/structure/types/manage/page/fields", click in "Add field", in "Re-use an existing field', select "Image: Field image", "Save and continue".
5- Bump!

RuntimeException: A config mapper can only contain configuration for a single language. in Drupal\config_translation\ConfigNamesMapper->getLangcode() (line 398 of core/modules/config_translation/src/ConfigNamesMapper.php).

Drupal\config_translation\Access\ConfigTranslationOverviewAccess->access(Object, Object)
call_user_func_array(Array, Array)
Drupal\Core\Access\AccessManager->performCheck('config_translation.access.overview', Object)
Drupal\Core\Access\AccessManager->check(Object, Object, NULL, 1)
Drupal\Core\Access\AccessManager->checkNamedRoute('entity.field_config.config_translation_overview.node', Array, Object, 1)
Drupal\Core\Menu\LocalTaskManager->getTasksBuild('entity.field_config.node_field_edit_form', Object)
Drupal\Core\Menu\LocalTaskManager->getLocalTasks('entity.field_config.node_field_edit_form', 0)
Drupal\Core\Menu\Plugin\Block\LocalTasksBlock->build()
Drupal\block\BlockViewBuilder::preRender(Array)
call_user_func('Drupal\block\BlockViewBuilder::preRender', Array)
Drupal\Core\Render\Renderer->doRender(Array)
Drupal\Core\Render\Renderer->doRender(Array, )
Drupal\Core\Render\Renderer->render(Array)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1)
__TwigTemplate_0f04497e0607dbb55ae785558deed68eacde76e3989f8c8e0779ccf1b6af549c->doDisplay(Array, Array)
Twig_Template->displayWithErrorHandling(Array, Array)
Twig_Template->display(Array)
Twig_Template->render(Array)
twig_render_template('core/themes/seven/templates/page.html.twig', Array)
Drupal\Core\Theme\ThemeManager->render('page', Array)
Drupal\Core\Render\Renderer->doRender(Array, )
Drupal\Core\Render\Renderer->render(Array)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1)
__TwigTemplate_16436f2456eb6e9f1287d658a3b71677eb9755e9eb4796bd9e27a1209ebeaf7f->doDisplay(Array, Array)
Twig_Template->displayWithErrorHandling(Array, Array)
Twig_Template->display(Array)
Twig_Template->render(Array)
twig_render_template('core/themes/classy/templates/layout/html.html.twig', Array)
Drupal\Core\Theme\ThemeManager->render('html', Array)
Drupal\Core\Render\Renderer->doRender(Array, )
Drupal\Core\Render\Renderer->render(Array)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}()
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)

Fields have two config objects,
the "Edit" tab is field.field... and
the "Field settings" the field.storage...

When installing in spanish, using standard profile, the field and storage config have langcode es.
After adding english, and enabling configuration translation module, and specifying en in the admin url, the field config (not storage) has the langcode en.
The translate tab, then errors because it expects all the config on the form to have the same source.

This is only a problem when config translation is enabled. (The field stuff is fine.)

Proposed resolution

We should "fail gracefully" by catching the exception in the UI and notifying the user about this in some way.
- Still show the translate tabs and everything (and make sure that no fatals bubble up, i.e. catch the exception)
- On the translation overview show a drupal_set_message(..., 'warning') with something like

The configuration files related to the node type Article have different source language codes and thus cannot be translated:
or
"Warning, the field configuration and field settings configuration have different source languages, and will not be able to be translated."

• node.type.article: Spanish
• core.base_field_override.node.title.article: Hungarian

We could do other things as well (maybe followups), ideas could be:
- Show some suggestion on how to fix this (maybe link to d.o)
- (no) Fix this automatically somehow
- Present a UI to choose which langcode to set for all the config files
- Still allow to translate some of the config files for which the langcode matches (and still show a warning)

Remaining tasks

User interface changes

Adding a warning.

API changes

??

Data model changes

No.