Too unspecific error message when mention limit is exceeded

[rwi]

When trying to send a note with an @@ mention an unspecific error message ('An error occurred') is shown, if the number of users to be notified exceeds the configured limit. Currently you have to look into the logs to find out what went wrong. It would be better to have a more descriptive error message so that an administrator can be informed. The admin could for example decide to increase the limit.