Error message "Unexpected server error" is displayed instead of "Cannot get authorization code.", if OAuth token configuration was incomplete.

Steps to reproduce:

1. Open Alerts -> Media types
2. Create media type:
   Type: Email
   Email provider: Generic SMTP
   SMTP server: smtp.gmail.com
   SMTP server port: 587
   Email: add any email
   Authentication -> OAuth
3. Click configure OAuth tokens
4. Add any values to mandatory fields (if doesn't have a previously configured Gmail account)
5. Click Update
6. Close the pop-up window that appeared and see the error message

Result:
"Unexpected server error"
See screenshot:

Expected:
Message "Cannot get authorization code." should be displayed. (Behavior from 7.4 version)
See screenshot: