If cit is run with an unknown tool, print usage rather than throwing an exception

cit.py

 #!/usr/bin/env python
 # Copyright (c) 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 
 """Wrapper for updating and calling infra.git tools.
 
 This tool does a two things:
 * Maintains a infra.git checkout pinned at "deployed" in the home dir
@@ skipping 81 matching lines @@
   for root, _, files in os.walk(starting):
     if '__main__.py' in files:
       infra_tools.append(root[len(starting)+1:].replace(os.path.sep, '.'))
   cipd = os.path.join(TARGET_DIR, 'infra', 'cipd')
   for fn in os.listdir(cipd):
     if is_exe(os.path.join(cipd, fn)):
       cipd_tools.append(fn)
   return (sorted(infra_tools), sorted(cipd_tools))
 
 
-def run(args):
-  if args:
-    tool_name = args[0]
-    # Check to see if it is a infra tool first.
-    infra_dir = os.path.join(
-      TARGET_DIR, 'infra', 'infra', 'tools', *tool_name.split('.'))
-    cipd_file = os.path.join(TARGET_DIR, 'infra', 'cipd', tool_name)
-    if sys.platform.startswith('win'):
-      cipd_file += '.exe'
-    if (os.path.isdir(infra_dir)
-        and os.path.isfile(os.path.join(infra_dir, '__main__.py'))):
-      cmd = [
-          sys.executable, os.path.join(TARGET_DIR, 'infra', 'run.py'),
-          'infra.tools.%s' % tool_name]
-    elif os.path.isfile(cipd_file) and is_exe(cipd_file):
-      cmd = [cipd_file]
-
-    # Add the remaining arguments.
-    cmd.extend(args[1:])
-    return subprocess.call(cmd)
-
+def usage():
   infra_tools, cipd_tools = get_available_tools()
   print """usage: cit.py <name of tool> [args for tool]
 
   Wrapper for maintaining and calling tools in:
     "infra.git/run.py infra.tools.*"
     "infra.git/cipd/*"
 
   Available infra tools are:"""
   for tool in infra_tools:
     print '  * %s' % tool
 
   print """
   Available cipd tools are:"""
   for tool in cipd_tools:
     print '  * %s' % tool
 
 
+def run(args):
+  if not args:
+    return usage()
+
+  tool_name = args[0]
+  # Check to see if it is a infra tool first.
+  infra_dir = os.path.join(
+    TARGET_DIR, 'infra', 'infra', 'tools', *tool_name.split('.'))
+  cipd_file = os.path.join(TARGET_DIR, 'infra', 'cipd', tool_name)
+  if sys.platform.startswith('win'):
+    cipd_file += '.exe'
+  if (os.path.isdir(infra_dir)
+      and os.path.isfile(os.path.join(infra_dir, '__main__.py'))):
+    cmd = [
+        sys.executable, os.path.join(TARGET_DIR, 'infra', 'run.py'),
+        'infra.tools.%s' % tool_name]
+  elif os.path.isfile(cipd_file) and is_exe(cipd_file):
+    cmd = [cipd_file]
+  else:
+    print >>sys.stderr, 'Unknown tool "%s"' % tool_name
+    return usage()
+
+  # Add the remaining arguments.
+  cmd.extend(args[1:])
+  return subprocess.call(cmd)
+
 
 def main():
   parser = argparse.ArgumentParser("Chrome Infrastructure CLI.")
   parser.add_argument('-b', '--infra-branch', default='deployed',
       help="The name of the 'infra' branch to use (default is %(default)s).")
   parser.add_argument('args', nargs=argparse.REMAINDER)
 
   args, extras = parser.parse_known_args()
   if args.args and args.args[0] == '--':
     args.args.pop(0)
   if extras:
     args.args = extras + args.args
 
   if need_to_update(args.infra_branch):
     ensure_infra(args.infra_branch)
   return run(args.args)
 
 if __name__ == '__main__':
   sys.exit(main())