Add a "message" field to wrapped barback exceptions.

pkg/barback/lib/src/errors.dart

Index: pkg/barback/lib/src/errors.dart
diff --git a/pkg/barback/lib/src/errors.dart b/pkg/barback/lib/src/errors.dart
index 0c2b5c5dd5e3eb33eceb424152b6665406607350..3488e31459528b3c9ac40c140715eb7b552774a4 100644
--- a/pkg/barback/lib/src/errors.dart
+++ b/pkg/barback/lib/src/errors.dart
@@ -115,14 +115,16 @@ abstract class _WrappedException implements BarbackException {
   final error;
   final Chain stackTrace;
 
+  String get message => "$_message: ${getErrorMessage(error)}";
+
+  String get _message;
+
   _WrappedException(error, StackTrace stackTrace)
       : this.error = error,
         this.stackTrace = _getChain(error, stackTrace);
 
-  String get _message;
-
   String toString() {
-    var result = "$_message: $error";
+    var result = message;
     if (stackTrace != null) result = "$result\n${stackTrace.terse}";
     return result;
   }