Add a Client class to json_rpc_2.

pkg/json_rpc_2/lib/src/exception.dart

Index: pkg/json_rpc_2/lib/src/exception.dart
diff --git a/pkg/json_rpc_2/lib/src/exception.dart b/pkg/json_rpc_2/lib/src/exception.dart
index be9c2d3e2e5ed3c025185eded901fd8fbcfc6a11..2fb0713286ec8b083ad7452176134fc41d7568c7 100644
--- a/pkg/json_rpc_2/lib/src/exception.dart
+++ b/pkg/json_rpc_2/lib/src/exception.dart
@@ -64,4 +64,11 @@ class RpcException implements Exception {
       'id': id
     };
   }
+
+  String toString() {
+    var prefix = "JSON-RPC error $code";
+    var errorName = error_code.name(code);
+    if (errorName != null) prefix += " ($errorName)";
+    return "$prefix: $message";
+  }
 }