Add log in fever api saving process (#2664)

Before, there was no user log when the fever api credential saving process
was failing. There was one though for the admin user but it did not appear
in the interface.
Now, there is a user log showing what the problem is. The admin log is still
there but catch only unknown errors.

See #2663
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f