Catch exceptions from failed notifications

This fixes udiskie's behaviour if no notification service is installed (see #42):

Previously, in this case an exception was raised when trying to show the
notification. The exception was caught in the main loop causing udiskie not
to crash altogether. With the new 'device_added' notifications this could
happen before trying to automount, hence making udiskie useless.

While technically, it is a user error to start udiskie with notifications
enabled without having a notification service installed, this is an error
too easy to be made and should be handled gracefully by udiskie.