Return non zero RC if any tasks fail

This is the other part of https://github.com/aws/aws-cli/pull/388

In this PR, I'm distinguishing between a precondition failure (for example, a destination bucket does not exist) which has an RC of 255 vs. a task execution failure (something that happens while the tasks are executing, say a network failure of disk space issue) which has an RC of 1.  Not sure if that distinction matters, but in the case of a task execution failure, it's likely that there's a partial failure.  This could happen if out of 100 files, only 1 file had an issue and failed to upload.

Not opposed to unifying the error RC and changing it to 255 though.