Print response from the server on HTTP errors from Rietveld

This will make explanation provided by Rietveld visible to the user, e.g.
"Request to https://codereview.chromium.org/1386443003/edit_flags failed: Cannot
set commit on an issue that does not have a commit queue".

R=phajdan.jr@chromium.org
BUG=529339

Review URL: https://codereview.chromium.org/1387833002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@297009 0039d316-1c4b-4281-b951-d872f2087c98

rietveld.py

@@ -449,6 +449,9 @@ class Rietveld(object):
             raise
         # If reaching this line, loop again. Uses a small backoff.
         time.sleep(min(10, 1+retry*2))
+    except urllib2.HTTPError as e:
+      print 'Request to %s failed: %s' % (e.geturl(), e.read())
+      raise
     finally:
       upload.ErrorExit = old_error_exit