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

Additional changes:
* repository.merge including exceptions is is extracted into a
separate method to make things clearer
* update! is used instead of update so we don't silently ignore
an error

Related to !41857