Add notification on unexpected exit

I believe that this fixes #36
Catch possible errors from notification commands
Remove the #exit on push failures

Unless I have missed something in the code, it seems that this exit
command would kill the entire daemon when there is a push error in
only one of the runners. That seems overkill.

However, I have added a TODO marker to go back and record status
information about the error. This could then be displayed through
either the web interface or the CLI status.

This should fix #47
Rescue any StandardErrors in #push_changes

The commands involved in the push and end up throwing errors that are
not critical, or will be resolved in the next check.
The method has been updated to rescue all StandardErrors and trigger a
notification. Similar to the previous commit, errors in a single share
should probably not kill the entire daemon.

Also similarly to the last commit these types of errors could probably
be recorded as status information on the share which was affected.

This should fix #53
Merge pull request #60 from acant/bug/unexpected_exit

Attempting to resolve some unexpected exits