Async-step, first cut.

runtime/observatory/lib/src/cli/command.dart

Index: runtime/observatory/lib/src/cli/command.dart
diff --git a/runtime/observatory/lib/src/cli/command.dart b/runtime/observatory/lib/src/cli/command.dart
index 0f573768314453149ba9965a1e90b83b1056b526..d6be00da44bcc246e6a1fc82dcc7130d20840583 100644
--- a/runtime/observatory/lib/src/cli/command.dart
+++ b/runtime/observatory/lib/src/cli/command.dart
@@ -170,12 +170,12 @@ class RootCommand extends _CommandBase {
     var commands =  _match(args, true);
     if (commands.isEmpty) {
       // TODO(turnidge): Add a proper exception class for this.
-      return new Future.error('notfound');
+      return new Future.error('No such command');
     } else if (commands.length == 1) {
       return commands[0].run(args.sublist(commands[0]._depth));
     } else {
       // TODO(turnidge): Add a proper exception class for this.
-      return new Future.error('ambiguous');
+      return new Future.error('Ambiguous command');
     }
   }