Allow mirrored repositories that have been moved or deleted to be removed

Use the "Mirror Bitbucket Team" built-in script to mirror some repositories from Bitbucket. Then delete those repositories in Bitbucket or move them to another project and go to the "Configured Mirrored Repositories" built-in script. Click on check to check the sync status of that repository and the following error message is shown:

This is not useful for an admin to verify why it failed if there are many mirrored repositories. We should log that the repository could not be found and remove the mirror from the configured ones.