Print a helpful error message when observatory_tool.py encounters an error

tools/observatory_tool.py

 #!/usr/bin/env python
 # Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 """Helper for building and deploying Observatory"""
 
 import argparse
 import os
 import platform
 import shutil
@@ skipping 21 matching lines @@
     'ShadowDOM.*',
     'webcomponents.*',
     'webcomponents-lite.js',
     'unittest*',
     '*_buildLogs*',
     '*.log',
     '*~')
 
 usage = """observatory_tool.py [options]"""
 
+def DisplayBootstrapWarning():
+  print """\
+
+WARNING: Your system cannot run the checked-in Dart SDK. Using the
+bootstrap Dart executable will make debug builds slow.
+Please see the Wiki for instructions on replacing the checked-in Dart SDK.
+
+https://github.com/dart-lang/sdk/wiki/The-checked-in-SDK-in-tools
+
+To use the dart_bootstrap binary please update the PubCommand function
+in the tools/observatory_tool.py script.
+
+"""
+
+def DisplayFailureMessage():
+  print """\
+
+ERROR: Observatory failed to build. What should you do?
+
+1. Revert to a working revision of the Dart VM
+2. Contact zra@, johnmccutchan@, dart-vm-team@
+3. File a bug: https://github.com/dart-lang/sdk/issues/new
+
+"""
+
 # Run |command|. If its return code is 0, return 0 and swallow its output.
 # If its return code is non-zero, emit its output unless |always_silent| is
 # True, and return the return code.
 def RunCommand(command, always_silent=False):
   try:
     # Dart IO respects the following environment variables to configure the
     # HttpClient proxy: https://api.dartlang.org/stable/1.22.1/dart-io/HttpClient/findProxyFromEnvironment.html
     # We strip these to avoid problems with pub build and transformers.
     no_http_proxy_env = os.environ.copy()
     no_http_proxy_env.pop('http_proxy', None)
     no_http_proxy_env.pop('HTTP_PROXY', None)
     no_http_proxy_env.pop('https_proxy', None)
     no_http_proxy_env.pop('HTTPS_PROXY', None)
     subprocess.check_output(command,
                             stderr=subprocess.STDOUT,
                             env=no_http_proxy_env)
     return 0
   except subprocess.CalledProcessError as e:
     if not always_silent:
       print ("Command failed: " + ' '.join(command) + "\n" +
               "output: " + e.output)
+      DisplayFailureMessage()
     return e.returncode
 
 def CreateTimestampFile(options):
   if options.stamp != '':
     dir_name = os.path.dirname(options.stamp)
     if dir_name != '':
       if not os.path.exists(dir_name):
         os.mkdir(dir_name)
     open(options.stamp, 'w').close()
 
@@ skipping 58 matching lines @@
     except OSError as e:
       pass
   options.pub_snapshot = None
 
   # We need a dart executable.
   return (options.dart_executable is not None)
 
 def ChangeDirectory(directory):
   os.chdir(directory);
 
-def DisplayBootstrapWarning():
-  print """\
-
-
-WARNING: Your system cannot run the checked-in Dart SDK. Using the
-bootstrap Dart executable will make debug builds slow.
-Please see the Wiki for instructions on replacing the checked-in Dart SDK.
-
-https://github.com/dart-lang/sdk/wiki/The-checked-in-SDK-in-tools
-
-To use the dart_bootstrap binary please update the PubCommand function
-in the tools/observatory_tool.py script.
-
-"""
-
 def PubCommand(dart_executable,
                pub_executable,
                pub_snapshot,
                command,
                silent):
   if pub_executable is not None:
     executable = [pub_executable]
   elif pub_snapshot is not None:
     executable = [utils.CheckedInSdkExecutable(), pub_snapshot]
   else:
@@ skipping 69 matching lines @@
   if (options.dart_executable != None):
     options.dart_executable = os.path.abspath(options.dart_executable)
   if (options.pub_executable != None):
     options.pub_executable = os.path.abspath(options.pub_executable)
   if (options.pub_snapshot != None):
     options.pub_snapshot = os.path.abspath(options.pub_snapshot)
   if (options.stamp != ''):
     options.stamp = os.path.abspath(options.stamp)
   if len(args) == 1:
     args[0] = os.path.abspath(args[0])
-  # Pub must be run from the project's root directory.
-  ChangeDirectory(options.directory)
-  result = ExecuteCommand(options, args)
-  if result == 0:
-    CreateTimestampFile(options)
-  return result
+  try:
+    # Pub must be run from the project's root directory.
+    ChangeDirectory(options.directory)
+    result = ExecuteCommand(options, args)
+    if result == 0:
+      CreateTimestampFile(options)
+    return result
+  except:
+    DisplayFailureMessage()
 
 
 if __name__ == '__main__':
   sys.exit(main());