When a file upload fails during "arc diff", show why

Summary: We currently swallow the exception message, but this isn't useful. Fixes T3354.

Test Plan:
  $ arc diff HEAD^ --conduit-uri=http://local.aphront.com:8080/ --only
  Uploading 1 files...
  Failed to upload new binary 'large.png'.

  [HTTP/500] Internal Server Error
  As received by the server, this request had a nonzero content length but no POST data.

  Normally, this indicates that it exceeds the 'post_max_size' setting in the PHP configuration on the server. Increase the 'post_max_size' setting or reduce the size of the request.

  Request size according to 'Content-Length' was '2093052', 'post_max_size' is set to '100K'.

      Continue? [Y/n]

Reviewers: jamesr, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T3354

Differential Revision: https://secure.phabricator.com/D6186