Add help to landmines.py if it errors out with unexpected builder

build/landmines.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 """
 This file holds a list of reasons why a particular build needs to be clobbered
 (or a list of 'landmines').
 
 This script runs every build as a hook. If it detects that the build should
@@ skipping 150 matching lines @@
         target + ('-iphoneos' if is_iphone else ''))
   elif build_tool == 'make':
     ret = os.path.join(SRC_DIR, 'out', target)
   elif build_tool == 'ninja':
     ret = os.path.join(SRC_DIR, 'out', target)
   elif build_tool in ['msvs', 'vs', 'ib']:
     ret = os.path.join(SRC_DIR, 'build', target)
   elif build_tool == 'scons':
     ret = os.path.join(SRC_DIR, 'sconsbuild', target)
   else:
-    raise NotImplementedError()
+    raise NotImplementedError('Unexpected GYP_GENERATORS (%s)' % build_tool)
   return os.path.abspath(ret)
 
 
 def set_up_landmines(target):
   """Does the work of setting, planting, and triggering landmines."""
   out_dir = get_target_build_dir(builder(), target, platform() == 'ios')
 
   landmines_path = os.path.join(out_dir, '.landmines')
   if not os.path.exists(out_dir):
     os.makedirs(out_dir)
@@ skipping 38 matching lines @@
   gyp_helper.apply_chromium_gyp_env()
 
   for target in ('Debug', 'Release'):
     set_up_landmines(target)
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())