Combine the ExitStatus and Err values from remote.Cmd into an error
returned by Wait, better matching the behavior of the os/exec package.

Non-zero exit codes are returned from Wait as a remote.ExitError.
Communicator related errors are returned directly.

Clean up all the error handling in the provisioners using a
communicator. Also remove the extra copyOutput synchronization that was
copied from package to package.

Since all use cases of ExitStatus are just putting it into fmt.Errorf,
usually with the command string, have ExitStatus do that for the caller.