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

lib/src/exceptions.dart

Index: lib/src/exceptions.dart
diff --git a/lib/src/formatter_exception.dart b/lib/src/exceptions.dart
similarity index 81%
rename from lib/src/formatter_exception.dart
rename to lib/src/exceptions.dart
index 6696aa4f35794ed50dfbf8cc5effd7fd9b9918dc..41756af06421fd43d7d7d66712626379dbbff14a 100644
--- a/lib/src/formatter_exception.dart
+++ b/lib/src/exceptions.dart
@@ -51,3 +51,22 @@ class FormatterException implements Exception {
 
   String toString() => message();
 }
+
+/// Exception thrown when the internal sanity check that only whitespace
+/// changes are made fails.
+class UnexpectedOutputException implements Exception {
+  /// The source being formatted.
+  final String _input;
+
+  /// The resulting output.
+  final String _output;
+
+  UnexpectedOutputException(this._input, this._output);
+
+  String toString() {
+    return """The formatter produced unexpected output. Input was:
+$_input
+Which formatted to:
+$_output""";
+  }
+}