Make skydb start url error out nicely

sky/tools/skydb

 #!/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 skypy.paths
 from skypy.skyserver import SkyServer
 import argparse
 import json
 import logging
@@ skipping 123 matching lines @@
 
     def _create_paths_for_build_dir(self, build_dir):
         # skypy.paths.Paths takes a root-relative build_dir argument. :(
         abs_build_dir = os.path.abspath(build_dir)
         root_relative_build_dir = os.path.relpath(abs_build_dir, SRC_ROOT)
         return skypy.paths.Paths(root_relative_build_dir)
 
     def start_command(self, args):
         # FIXME: Lame that we use self for a command-specific variable.
         self.paths = self._create_paths_for_build_dir(args.build_dir)
-
         self.stop_command(None) # Quit any existing process.
         self.pids = {} # Clear out our pid file.
 
+        if not os.path.exists(self.paths.mojo_shell_path):
+            print "mojo_shell not found in build_dir '%s'" % args.build_dir
+            print "Are you sure you sure that's a valid build_dir location?"
+            print "See skydb start --help for more info"
+            sys.exit(2)
+
         # FIXME: This is probably not the right way to compute is_android
         # from the build directory?
         gn_args = gn_args_from_build_dir(args.build_dir)
         is_android = 'android_sdk_version' in gn_args
 
         sky_server = self.sky_server_for_args(args)
         self.pids['sky_server_pid'] = sky_server.start()
         self.pids['sky_server_port'] = sky_server.port
         self.pids['sky_server_root'] = sky_server.root
 
@@ skipping 216 matching lines @@
         load_parser.set_defaults(func=self.load_command)
 
         args = parser.parse_args()
         args.func(args)
 
         self._write_pid_file(PID_FILE_PATH, self.pids)
 
 
 if __name__ == '__main__':
     SkyDebugger().main()