Fix upload bug.

Fixed bug where the ChunkedReader object and swift_client.put_object()
would get out of sync about how many bytes they had left. When there
were retries, the same ChunkedReader object was getting reused, so it
would end early.  Now a ChunkedReader always yields data with its read()
of the size requested.

Tracebacks for exceptions in the worker are now included in the results
and printed by the master at the DEBUG loglevel.