More gracefully handle browser errors.

lib/src/runner/browser/content_shell.dart

 // 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.
 
 library test.runner.browser.content_shell;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
+import 'package:stack_trace/stack_trace.dart';
+
+import '../../utils.dart';
 import '../application_exception.dart';
 import 'browser.dart';
 
 /// A converter that transforms a byte stream into a stream of lines.
 final _lines = UTF8.decoder.fuse(const LineSplitter());
 
 /// A class for running an instance of the Dartium content shell.
 ///
 /// Most of the communication with the browser is expected to happen via HTTP,
 /// so this exposes a bare-bones API. The browser starts as soon as the class is
@@ skipping 43 matching lines @@
         // TODO(nweiz): link to dartlang.org once it has download links for
         // content shell
         // (https://github.com/dart-lang/www.dartlang.org/issues/1164).
         throw new ApplicationException(
             "You're using an expired content_shell. Upgrade to the latest "
                 "version:\n"
             "http://gsdview.appspot.com/dart-archive/channels/stable/release/"
                 "latest/dartium/");
       }
 
-      if (exitCode != 0) throw "Content shell failed with exit code $exitCode.";
-    }).then(_onExitCompleter.complete)
-        .catchError(_onExitCompleter.completeError);
+      if (exitCode == 0) return null;
+
+
+      return UTF8.decodeStream(_process.stderr).then((error) {
+        throw new ApplicationException(
+            "Content shell failed with exit code $exitCode:\n$error");
+      });
+    }).then(_onExitCompleter.complete).catchError((error, stackTrace) {
+      if (stackTrace == null) stackTrace = new Trace.current();
+      _onExitCompleter.completeError(
+          new ApplicationException(
+              "Failed to start content shell: ${getErrorMessage(error)}."),
+          stackTrace);
+    });
   }
 
   Future close() {
     _onProcessStarted.then((_) => _process.kill());
 
     // Swallow exceptions. The user should explicitly use [onExit] for these.
     return onExit.catchError((_) {});
   }
 
   /// Return the default executable for the current operating system.
   String _defaultExecutable() =>
       Platform.isWindows ? "content_shell.exe" : "content_shell";
 }