Add a check when people specify wrong change list name to give a meaningful message.

I saw that in the exceptions and the current error (file not found) is not useful.

TEST=none
BUG=none

Review URL: http://codereview.chromium.org/3332017

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@58978 0039d316-1c4b-4281-b951-d872f2087c98

gcl.py

@@ -959,6 +959,8 @@ def CMDchange(args):
               "directory.")
 
   if len(args) == 2:
+    if not os.path.isfile(args[1]):
+      ErrorExit('The change "%s" doesn\'t exist.' % args[1])
     f = open(args[1], 'rU')
     override_description = f.read()
     f.close()