[dart] change FidlError message format

* $RuntimeType: $Message is the usual format in Dart, e.g., https://github.com/dart-lang/sdk/blob/5721d8af6d8755fa6f70f3951de2c451c57bf12f/sdk/lib/core/exceptions.dart#L32
* that makes it easier to parse from the C++ runners if they all have the same format

TESTED=triggered FidlError, correctly parsed in crash analyzer

Change-Id: Ic1fae9dab3f5c6e6d3317a9efddf8bb249e38038
diff --git a/public/dart/fidl/lib/src/error.dart b/public/dart/fidl/lib/src/error.dart
index be7027c..70b0b42 100644
--- a/public/dart/fidl/lib/src/error.dart
+++ b/public/dart/fidl/lib/src/error.dart
@@ -10,5 +10,5 @@
   final String message;
 
   @override
-  String toString() => 'FidlError($message)';
+  String toString() => 'FidlError: $message';
 }