Handle error messages in Mounter uniformly
Add notifications for failed jobs

I do not stand a hundred percent behind the the path taken in this
commit: It implies that with udisks1 error messages are available only
by caching exception messages. This works only for jobs that were
initiated through a Mounter using the very same Daemon object.
Add 'Retry' button to fail notifications
Remove automatic retries to unlock LUKS partitions

Being stuck in the retry loop makes it impossible to get the
job-failed/job-completed events for UDisks1 right: see the removed
comment in udisks1.py.
Update CHANGELOG
Merge branch 'job-failed-notification'