Issue #3276720: Add ability to implement custom error handler

• Adds onError property to the DrupalState class that allows for custom function to handle thrown errors
• Adds some better error messages for when Drupal responds to a fetch with 404 (see the fetch wrappers fetchApiIndex and fetchJsonapiEndpoint
• Adds tests for the custom onError handler and the new error messages

If the original error messages from Drupal are needed, we can add a flag so that the default is to not catch or enhance any errors coming from Drupal. The reason I did not do it this way to start is that the errors from Drupal were not helpful (for example: 'unexpected token < in JSON position 2'. This doesn't seem helpful to me, but 'Tried to fetch example.com/not-valid. Status code: 404' seems better. Thoughts?)

I also decided to wait on trying to implement onErrorRetry until this portion of the feature was reviewed.

Closes #3276720