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Refactor common code to _add_action function
Catch exceptions when trying to show notifications

..and simply log the error to the console.

This fixes udiskie's behaviour if no notification service is installed:

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.
Add comment for bug fix
Inline _create_notification function
Move another bug related comment in the right place
Merge pull request #43 from coldfix/handle-notify-exception

Catch exceptions from failed notifications