Save and expose only generic merge error

What does this MR do?

When an error occurs during merge, the error message is exposed to user and it is also saved in DB. This error message may be user unfriendly (as in !41820) and it could also expose a detailed backend information.

Instead of displaying the specific error message, only sanitized generic message is displayed.

Are there points in the code the reviewer needs to double check?

This is potentially controversial change because disadvantage is that user doesn't get specific reason of failure.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug

What are the relevant issue numbers?

Closes #41857 (closed)