Explain empty report in compare_codereview.py.

tools/compare_codereview.py

 #!/usr/bin/python2
 
 # Copyright 2014 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Skia's Chromium Codereview Comparison Script.
 
 This script takes two Codereview URLs, looks at the trybot results for
@@ skipping 307 matching lines @@
 
     Args:
         control_url, roll_url: (strings) URL of the format
             https://codereview.chromium.org/?????????
 
         verbosity: (int) verbose level.  0, 1, or 2.
     """
     # pylint: disable=I0011,R0914,R0912
     control = CodeReviewHTMLParser.parse(control_url)
     roll = CodeReviewHTMLParser.parse(roll_url)
-    if not (control and roll):
+    all_bots = set(control) & set(roll)  # Set intersection.
+    if not all_bots:
+        print >> sys.stderr, (
+            'Error:  control %s and roll %s have no common trybots.'
+            % (list(control), list(roll)))
         return
 
     control_name = '[control %s]' % control_url.split('/')[-1]
     roll_name = '[roll %s]' % roll_url.split('/')[-1]
-    all_bots = set(control) & set(roll)  # Set intersection.
 
     out = sys.stdout
     if verbosity > 0:
         # Print out summary of all of the bots.
         out.write('%11s %11s %4s %s\n\n' %
                   ('CONTROL', 'ROLL', 'DIFF', 'BOT'))
         for bot in sorted(all_bots):
             if control[bot].status != roll[bot].status:
                 diff = '****'
             elif (control[bot].status != 'success' or
@@ skipping 34 matching lines @@
         out.write('\n')
 
 
 if __name__ == '__main__':
     if len(sys.argv) < 3:
         print >> sys.stderr, __doc__
         exit(1)
     main(sys.argv[1], sys.argv[2],
          int(os.environ.get('COMPARE_CODEREVIEW_VERBOSITY', 1)))