Validate input dates in my_reviews.py.

Review URL: https://codereview.chromium.org/194753007

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@256292 0039d316-1c4b-4281-b951-d872f2087c98

my_reviews.py

@@ -342,6 +342,14 @@ def main():
     options.end = end
     print >> sys.stderr, 'Using range %s to %s' % (
         options.begin, options.end)
+
+  # Validate dates.
+  try:
+    to_datetime(options.begin)
+    to_datetime(options.end)
+  except ValueError as e:
+    parser.error('%s: %s - %s' % (e, options.begin, options.end))
+
   if options.count:
     print_count(
         options.reviewer,