Expose full set of debugging data in migrate_message table (filterable/searchable)

Problem/Motivation

There's a great deal of info about what actually went wrong when you performed a migration, but it's all kept in a table called migrate_message with no mechanism (UI, command-line prompt, etc.) to extract info. Ideally, there would be a searchable/filterable view of this information.

Proposed resolution

Add a new page at admin/reports/upgrade-messages with a summary, and child pages that display the messages in any migrate_message table. Add the ability to filter on the migration and the severity.

The new pages call the fields() method of the related source plugin. Some of these need to be updated to catch exceptions when the source database has not been configured:

• MenuLink.php (There are also a lot of uses of the global t() in fields().)
• Menu.php
• d7/User.php
• d6/User.php (in baseFields())
• d6/ProfileFieldValues.php

Work-around: There is a drush command to view the messages, drush mmsg migration-name available from migrate_tools and migrate_run.

No message tables

With Message tables

D7_menu

Remaining tasks

Review
Agree on wording for the explanation of 'Not available' in some columns.
Agree on names that is align with the changes in #2713327: Provide a way to remove migration tables (ID map etc.).
Potential new issue for the changes here in Menu.php

User interface changes

New form for displaying messages from the migrate_message tables.

API changes

Data model changes

Release notes snippet