Throw a useful error if there is a problem communicating with the pro…

…tocol

lib/src/devtools.dart

@@ -174,6 +174,18 @@ class _VMWebsocketConnection {
       return;
     }
 
+    var completer = _pendingRequests.remove(id);
+    if (completer == null) {
+      _log.severe('Failed to pair response with request');
+    }
+
+    var error = json['error'];
+    if (error != null) {
+      var errorObj = new JsonRpcError.fromJson(error);
+      completer.completeError(errorObj);
+      return;
+    }
+
     var innerResponse = json['result'];
     if (innerResponse == null) {
       // Support for 1.9.0 <= vm version < 1.10.0.
@@ -191,10 +203,38 @@ class _VMWebsocketConnection {
         message = JSON.decode(innerResponse);
       }
     }
-    var completer = _pendingRequests.remove(id);
-    if (completer == null) {
-      _log.severe('Failed to pair response with request');
-    }
     completer.complete(message);
   }
 }
+
+class JsonRpcError extends Error {
+  final int code;
+  final String message;
+  final data;
+
+  // http://www.jsonrpc.org/specification
+  // -32601	Method not found	The method does not exist / is not available.
+  bool get isMethodNotFound => code == -32601;
+
+  JsonRpcError(this.code, this.message, this.data);
+
+  factory JsonRpcError.fromJson(Map<String, dynamic> json) =>
+      new JsonRpcError(json['code'], json['message'], json['data']);
+
+  String toString() {
+    var msg = 'JsonRpcError: $message';
+    if (isMethodNotFound) {
+      if (data is Map) {
+        var request = data['request'];
+        if (request is Map) {
+          var method = request['method'];
+          if (method != null) {
+            msg = '$msg - "$method"';
+          }
+        }
+      }
+    }
+
+    return '$msg ($code)';
+  }
+}