Produce error if viewing a media without a source file (Issue-2110)

[seth-shaw-unlv]

GitHub Issue: Islandora/documentation#2110

What does this Pull Request do?

Produces an error message instead of a WSOD when viewing a media where the source file is absent and the fedora URI is set to display.

What's new?

Adds a check to islandora_entity_view around the source file to throw an error and return early if it wasn't found.

• Does this change add any new dependencies? No.
• Does this change require any other modifications to be made to the repository
  (i.e. Regeneration activity, etc.)? No.
• Could this change impact execution of existing code? Makes it not crash.

How should this be tested?

• Create a media without a source file (this occurred with a broken migration).
  • Create a media via the UI
  • Jump to the command-line and the interactive terminial drush php:cli
  • Load the media, e.g. $m = \Drupal::entityTypeManager()->getStorage('media')->load(1); (replacing "1" with your test media's ID)
  • Get the source file: $f = \Drupal::service('islandora.media_source_service')->getSourceFile($m);
  • Delete it: $f->delete();
  • Clear your cache
• Make sure the field_gemini_uri is set to display with the media's default mode
• Visit the media's page and receive a WSOD
• Apply the PR
• Visit the media's page again and NOT WSOD.
• Check the logs to see appropriate error message.

Interested parties

@Islandora/committers

[jordandukart]

With #869 merged going to re-run tests for sanity's sake but will review this.

[jordandukart]

Does as it says on the tin, 👍.