Explain empty report in compare_codereview.py.

Explain empty report in compare_codereview.py.

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

BUG=skia:

Committed: http://code.google.com/p/skia/source/detail?r=13194

[borenet, 2014-01-24 15:41:54]

Can you upload this again?  I can't see the diff.

[hal.canary, 2014-01-27 14:36:47]

On 2014/01/24 15:41:54, borenet wrote:
> Can you upload this again?  I can't see the diff.

done

[borenet, 2014-01-27 14:37:43]

On 2014/01/27 14:36:47, Hal Canary wrote:
> On 2014/01/24 15:41:54, borenet wrote:
> > Can you upload this again?  I can't see the diff.
> 
> done

Thanks.  LGTM

[commit-bot: I haz the power, 2014-01-27 15:45:34]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/halcanary@google.com/135683003/60001

[commit-bot: I haz the power, 2014-01-27 16:34:48]

Change committed as 13194