Fix DocumentsUI error-reporting code for handling copy errors.

- Correctly handle the case when opening the src file fails.

- Add logging for easier debugging.

- Add the transfer mode extra so that the failure dialog displays the
  correct message, and retries in the correct mode.

BUG=23414397

Change-Id: I3bba11ed393678d614779d9b687e7e123c4fb618
1 file changed