Explain empty report in compare_codereview.py.

Let user know when control and roll have no common trybots, instead of
being silent.

BUG=skia:
R=borenet@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/135683003

git-svn-id: http://skia.googlecode.com/svn/trunk@13194 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed