SyncTask repeats auth exception for every source file

[MichaelHackett]

Right now, the SyncTask spins up a new UploadTask subtask for each individual file, and schedules them all at once, up front. If the AWS credentials happen to be bad, there will be an exception generated for each file, possibly hundreds or thousands of them.

It would be nice if the SyncTask either did a pre-check on the AWS credentials, or else makes the per-file upload tasks somehow dependent on earlier ones, so that later tasks will be automatically aborted. (It would be fine to have a small number that are started simultaneously to all fail, but all subsequently started tasks should immediately and silently stop themselves.)

(I'm not sure which is the better approach, and have no experience with the AWS SDK and ExecutorService, but I'll have a look if I get time later. I just wanted to get this in, and maybe get feedback on how to proceed.)