codereview: warn about 'hg upload' not uploading

This seems to be what causes the 'chunk mismatch' errors.
I don't know why the server tells us there's nothing to upload,
but at the least we can warn that it did, so that the user
will try again.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/62840043

lib/codereview/codereview.py

@@ -367,6 +367,8 @@ class CL(object):
 			msg = lines[0]
 			patchset = lines[1].strip()
 			patches = [x.split(" ", 1) for x in lines[2:]]
+		else:
+			print >>sys.stderr, "Server says there is nothing to upload (probably wrong):\n" + msg
 		if response_body.startswith("Issue updated.") and quiet:
 			pass
 		else: