Print command and stdout/err on error

The stdout and stderr (which are merged) are now read
from the created Process instance if the return code
is not 0.
Silently ignore ObjectNotExistException

If `checkLocation` throws an exception, then
`cleanupUpload` will definitely throw an ONEE
since the proxy no longer exists. Cleaner would
be to null some instance variable, but due to
the possibly multi-threaded nature of the Transfer
object, we're doing something simpler here.