Don't spawn debugger.mojo by default in `mojo_run`.

mojo/devtools/common/mojo_debug

 #!/usr/bin/env python
 # Copyright 2014 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.
 
 import argparse
 import codecs
 import logging
 import os.path
 import requests
@@ skipping 27 matching lines @@
 
 def _send_request(request, payload=None):
   """Sends a request to mojo:debugger."""
   try:
     url = 'http://localhost:%s/%s' % (_MOJO_DEBUGGER_PORT, request)
     if payload:
       return requests.post(url, payload)
     else:
       return requests.get(url)
   except requests.exceptions.ConnectionError:
-    print 'Failed to connect to mojo:debugger, make sure the shell is running.'
+    print ('Failed to connect to debugger.mojo. Make sure the shell is running '
+           'and the app was started with debugger, ie. through '
+           '`mojo_run --debugger APP_URL`')
+
     return None
 
 
 def _tracing_start(_):
   """Starts tracing."""
   if not _send_request('start_tracing'):
     return 1
   print "Started tracing."
   return 0
 
@@ skipping 25 matching lines @@
     trace_file.write('{"traceEvents":[')
     trace_file.write(response.content)
     trace_file.write(']}')
   print "Trace saved in %s" % file_name
   return 0
 
 
 def _add_tracing_command(subparsers):
   """Sets up the command line parser to manage tracing."""
   tracing_parser = subparsers.add_parser('tracing',
-      help='trace event profiler')
+      help='tracer (requires debugger.mojo)')
   tracing_subparser = tracing_parser.add_subparsers(
       help='the command to run')
 
   start_tracing_parser = tracing_subparser.add_parser('start',
       help='start tracing')
   start_tracing_parser.set_defaults(func=_tracing_start)
 
   stop_tracing_parser = tracing_subparser.add_parser('stop',
       help='stop tracing and retrieve the result')
   stop_tracing_parser.add_argument('file_name', type=str, nargs='?',
       help='name of the output file (optional)')
   stop_tracing_parser.set_defaults(func=_tracing_stop)
 
 
 def _wm_load(args):
   """Loads (embeds) the given url in the window manager."""
   if not _send_request('load', args.url):
     return 1
   return 0
 
 
 def _add_wm_command(subparsers):
   """Sets up the parser for the 'wm' command."""
-  wm_parser = subparsers.add_parser('wm', help='window manager')
+  wm_parser = subparsers.add_parser('wm', help='window manager (requires '
+                                    'debugger.mojo)')
   wm_subparser = wm_parser.add_subparsers(
       help='the command to run')
 
   wm_load_parser = wm_subparser.add_parser('load',
       help='load (embed) the given url')
   wm_load_parser.add_argument('url', type=str,
       help='the url to load')
   wm_load_parser.set_defaults(func=_wm_load)
 
 
@@ skipping 192 matching lines @@
   _add_device_command(subparsers)
   _add_tracing_command(subparsers)
   _add_wm_command(subparsers)
   _add_gdb_command(subparsers)
 
   args = parser.parse_args()
   return args.func(args)
 
 if __name__ == '__main__':
   sys.exit(main())