Don't dump a stack trace on the user if the formatter had bad output.

bin/format.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.
 
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:args/args.dart';
 
 import 'package:dart_style/src/dart_formatter.dart';
-import 'package:dart_style/src/formatter_exception.dart';
+import 'package:dart_style/src/exceptions.dart';
 import 'package:dart_style/src/formatter_options.dart';
 import 'package:dart_style/src/io.dart';
 import 'package:dart_style/src/source_code.dart';
 
 // Note: The following line of code is modified by tool/grind.dart.
 const version = "1.0.3";
 
 void main(List<String> args) {
   var parser = new ArgParser(allowTrailingOptions: true);
 
@@ skipping 228 matching lines @@
     output = stdout;
   }
 
   output.write("""$message
 
 Usage: dartfmt [-n|-w] [files or directories...]
 
 ${parser.usage}
 """);
 }