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

lib/src/dart_formatter.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.
 
 library dart_style.src.dart_formatter;
 
 import 'dart:math' as math;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/src/dart/scanner/reader.dart';
 import 'package:analyzer/src/dart/scanner/scanner.dart';
 import 'package:analyzer/src/generated/parser.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/string_source.dart';
 
 import 'error_listener.dart';
-import 'formatter_exception.dart';
+import 'exceptions.dart';
 import 'source_code.dart';
 import 'source_visitor.dart';
 import 'string_compare.dart' as string_compare;
 
 /// Dart source code formatter.
 class DartFormatter {
   /// The string that newlines should use.
   ///
   /// If not explicitly provided, this is inferred from the source text. If the
   /// first newline is `\r\n` (Windows), it will use that. Otherwise, it uses
@@ skipping 95 matching lines @@
         throw new FormatterException([error]);
       }
     }
 
     errorListener.throwIfErrors();
 
     // Format it.
     var visitor = new SourceVisitor(this, lineInfo, source);
     var output = visitor.run(node);
     if (!string_compare.equalIgnoringWhitespace(source.text, output.text)) {
-      throw new FormatException('Cannot safely format file as formatting '
-          'would cause non-whitespace change(s).');
+      throw new UnexpectedOutputException(source.text, output.text);
     }
 
     return output;
   }
 }