Error messages when compiling twig with calls to theme() have a wrong context

#1880100-48: Allow to configure the h3 element for view style plugins tells something about a missing variable in views-view.html.twig, but the variable was actually missing in a different template.

Steps to reproduce:

1. Clean install of Drupal 8.
2. Try to print a template in bartik/templates/node.html.twig that does not exist - {{ badvariable }}
3. Clear caches.
4. Visit /node - you should see an exception like the following:

User warning: badvariable could not be found in _context in "core/themes/bartik/templates/page.html.twig" at line 170 in Drupal\Core\Template\TwigTemplate->getContextReference() (line 51 of core/lib/Drupal/Core/Template/TwigTemplate.php).

Since this exception is not coming from upstream Twig internals we should be able to fix it here, and maybe improve the error message while we're at it.