Update dartfmt message to be less confusing

R=tandrii@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1532563002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@298047 0039d316-1c4b-4281-b951-d872f2087c98

git_cl.py

@@ -3569,8 +3569,9 @@ def CMDformat(parser, args):
       if opts.dry_run and stdout:
         return_value = 2
     except dart_format.NotFoundError as e:
-      print ('Unable to check dart code formatting. Dart SDK is not in ' +
-             'this checkout.')
+      print ('Warning: Unable to check Dart code formatting. Dart SDK not ' +
+             'found in this checkout. Files in other languages are still ' +
+             'formatted.')
 
   return return_value