Inform user on submission if string was not saved remotely

Currently we save errors into the watchdog when the remote submission did not work, but do not inform the user. It can still happen that someone generally has the permission on the server to use remote submission, but is not a member of the groups it is being submitted to, which overrides the general permissions for good reasons. So we should inform users when a remote submission error happens. There is also that case of course that the server is not up, etc. Basically, the l10n_client_submit_translation() code should somehow communicate errors back to the JS UI.