Ensure apidoc exits with non-zero exit code when dart2js crashes.

dart/sdk/lib/_internal/dartdoc/bin/dartdoc.dart

 // Copyright (c) 2012, 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.
 
 /**
  * To generate docs for a library, run this script with the path to an
  * entrypoint .dart file, like:
  *
  *     $ dart dartdoc.dart foo.dart
  *
@@ skipping 233 matching lines @@
         copyDirectory(scriptDir.append('../static'), dartdoc.outputDir)
       ]);
     })
     .then((_) {
       print(dartdoc.status);
       if (dartdoc.totals == 0) {
         exit(1);
       }
     })
     .catchError((e) {
-        print('Error: generation failed: ${e.error}');
+        print('Error: generation failed: ${e}');
         exit(1);
     })
     .whenComplete(() => dartdoc.cleanup());
 }