More gracefully handle browser errors.

lib/src/runner/browser/phantom_js.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.phantom_js;
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
+import 'package:stack_trace/stack_trace.dart';
 
 import '../../util/exit_codes.dart' as exit_codes;
 import '../../util/io.dart';
+import '../../utils.dart';
 import '../application_exception.dart';
 import 'browser.dart';
 
 /// The PhantomJS script that opens the host page.
 final _script = """
 var system = require('system');
 var page = require('webpage').create();
 
 // PhantomJS versions older than 2.0.0 don't support the latest WebSocket spec.
 if (phantom.version.major < 2) phantom.exit(${exit_codes.protocol});
@@ skipping 51 matching lines @@
         _process = process;
         _onProcessStartedCompleter.complete();
 
         return _process.exitCode;
       });
     }).then((exitCode) {
       if (exitCode == exit_codes.protocol) {
         throw new ApplicationException(
             "Only PhantomJS version 2.0.0 or greater is supported.");
       }
-      if (exitCode != 0) throw "PhantomJS 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(
+            "PhantomJS 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 PhantomJS: ${getErrorMessage(error)}."),
+          stackTrace);
+    });
   }
 
   Future close() {
     _onProcessStarted.then((_) => _process.kill());
 
     // Swallow exceptions. The user should explicitly use [onExit] for these.
     return onExit.catchError((_) {});
   }
 }