Uploader crashes if any input directory does not exist or is inaccessible

[dfornika]

We've recently run into a condition where one of the input directories listed in the config inputdirs was inaccessible because it had become unmapped by Windows. When that happened, the uploader crashed, affecting both sequencers that were attached to the uploader process (not just the one whose output directory had become inaccessible).

It would be preferable if that exception were caught and handled, instead of the program crashing. That way, uploads could proceed for any sequencers whose inputdir was still accessible. An error should be logged when one of the inputdirs is inaccessible.