Add a "message" field to wrapped barback exceptions.

pkg/barback/lib/src/utils.dart

Index: pkg/barback/lib/src/utils.dart
diff --git a/pkg/barback/lib/src/utils.dart b/pkg/barback/lib/src/utils.dart
index d5a9fb62dd1130684c73390fd2904d7f7ccafbff..c26c7e8f0cbebd1f4117627d9bb8d3553f1900d5 100644
--- a/pkg/barback/lib/src/utils.dart
+++ b/pkg/barback/lib/src/utils.dart
@@ -325,3 +325,14 @@ Stream broadcastToSingleSubscription(Stream broadcast) {
       onDone: controller.close);
   return controller.stream;
 }
+
+/// A regular expression to match the exception prefix that some exceptions'
+/// [Object.toString] values contain.
+final _exceptionPrefix = new RegExp(r'^([A-Z][a-zA-Z]*)?(Exception|Error): ');
+
+/// Get a string description of an exception.
+///
+/// Many exceptions include the exception class name at the beginning of their
+/// [toString], so we remove that if it exists.
+String getErrorMessage(error) =>
+  error.toString().replaceFirst(_exceptionPrefix, '');