I recently fat-fingered the file I was editing s.t. the checkout I was in had no change from its upstream branch when I did "git cl upload".  The error message did give a reasonable clue as to what was going on, but could have been a fair ways more specific:

...
Presubmit checks passed.
Waiting for Emacs...
No output from ['git', 'diff', '--no-color', '--no-ext-diff', '--full-index', '--ignore-submodules', '--src-prefix=a/', '--dst-prefix=b/', '5d0334b8dba68367e4fea3bbd49993172ea0a0fc', 'HEAD']

Got exception while uploading -- saving description to /Users/rdsmith/.git_cl_description_backup

Not a high priority, but I suspect this'll be true for gerrit as well, so I thought I'd at least mention it.