Friendlier threaded problem handling

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Problems in threads like the ones happening in ME's remove_dead_tracks could be better handled than by simply throwing a crash report. The problem however is to modify jobprogress's capabilities to allow for some kind of return value in job_completed() to allow for, for example, a call to show_message() or something, a call that cannot happen in the async function.

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In the process of implementing this, I uncovered a bug which I filed at #196.

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(from [f75db44f5c07]) [#196 state:fixed] [#188] Fixed a crash on crash reporting under Cocoa and improved job management to allow threaded error handling on _job_finished().
https://bitbucket.org/hsoft/dupeguru/changeset/f75db44f5c07/

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(from [d99996484950]) [#188 state:fixed] Instead of showing a crash report on iTunes communication problems, show a friendlier message.
https://bitbucket.org/hsoft/dupeguru/changeset/d99996484950/