5182: Improve error reporting in uploader.

5182: Improve error reporting in uploader.

Missing CORS headers (and network errors which force the browser to
assume CORS headers are missing) are reported as error==="". In place
of the enigmatic "error:", we show a message hinting at network/CORS
problems and pointing the user to the browser debug console for
further clues.

Mixed-content errors (https://workbench/*.js attempts AJAX request at
http://proxy/*) don't invoke success/fail handlers at all, so we catch
them ahead of time and show an appropriate message.