Improve showing error message when merging two annotations failed

[MichaelBuessemeyer]

Context

When merging annotation fails with an error message, the concrete message from the backend can only be viewed in the expandable part of the error toast.

Expected Behavior

It would be nice to see the error message as part of the main toast text.

Current Behavior

If possible, it would be nice to propagate the error properly to show it to the user in an understandable way. The message is already clear, but too hidden for the user. But I think this is unrelated -> maybe open an issue / pr for this

Steps to Reproduce the bug

1. Create 2 annotations. One with proofreading the other without
2. try to merge them and see the error message

Potential enhancement initially identified in #7917 (review)