Added error handling and a better error message if we fail to write output in docgen.

pkg/docgen/lib/src/viewer.dart

 // Copyright (c) 2014, 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.
 
 /// Convenience methods wrapped up in a class to pull down the docgen viewer for
 /// a viewable website, and start up a server for viewing.
 library docgen.viewer;
 
 import 'dart:io';
 
@@ skipping 110 matching lines @@
       gen.outputDirectory);
   var webDocsDir = new Directory(path.join(_dartdocViewerDir.path, 'client',
       'web', 'docs'));
   if (dir.existsSync()) {
     // Move the docs folder to dartdoc-viewer/client/web/docs
     dir.renameSync(webDocsDir.path);
   }
 
   if (webDocsDir.existsSync()) {
     // Compile the code to JavaScript so we can run on any browser.
-    print('Compile app to JavaScript for viewing.');
+    print('Compiling the app to JavaScript.');
     var processResult = Process.runSync(gen.dartBinary, ['deploy.dart'],
         workingDirectory: path.join(_dartdocViewerDir.path, 'client'),
         runInShell: true);
     print('process output: ${processResult.stdout}');
     print('process stderr: ${processResult.stderr}');
     _runServer();
   }
 }
 
 void _compile() {
   if (_webDocsDir.existsSync()) {
     // Compile the code to JavaScript so we can run on any browser.
-    print('Compile app to JavaScript for viewing.');
+    print('Compiling the app to JavaScript.');
     var processResult = Process.runSync(gen.dartBinary, ['deploy.dart'],
         workingDirectory: _viewerCodePath, runInShell: true);
     print('process output: ${processResult.stdout}');
     print('process stderr: ${processResult.stderr}');
     var outputDir = path.join(_viewerCodePath, 'out', 'web');
     print('Docs are available at $outputDir');
   }
 }
 
 /// A simple HTTP server. Implemented here because this is part of the SDK,
@@ skipping 37 matching lines @@
         // Serve up file contents.
         file.openRead().pipe(response).catchError((e) {
           print('HttpServer: error while closing the response stream $e');
         });
       }
     }, onError: (e) {
       print('HttpServer: an error occured $e');
     });
   });
 }