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

tools/observatory_tool.py

Index: tools/observatory_tool.py
diff --git a/tools/observatory_tool.py b/tools/observatory_tool.py
index 5936b8220517fe4c9c8da69a42d5724d44b3dec2..57b3a067cf41683123224a40bc25ecbb596339c9 100755
--- a/tools/observatory_tool.py
+++ b/tools/observatory_tool.py
@@ -39,6 +39,31 @@ IGNORE_PATTERNS = shutil.ignore_patterns(
 
 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.
@@ -60,6 +85,7 @@ def RunCommand(command, always_silent=False):
     if not always_silent:
       print ("Command failed: " + ' '.join(command) + "\n" +
               "output: " + e.output)
+      DisplayFailureMessage()
     return e.returncode
 
 def CreateTimestampFile(options):
@@ -138,21 +164,6 @@ def ProcessOptions(options, args):
 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,
@@ -242,12 +253,15 @@ def main():
     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__':