Fix for an error in the error handler.

We pass exception as String message.

We would have caught it if we had run the server in the checked mode.
Instead it failed at the JSON serialization stage.

R=brianwilkerson@google.com
BUG=

Review URL: https://codereview.chromium.org//698563003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@41450 260f80e4-7a28-3924-810f-c04153c831b5

pkg/analysis_server/lib/src/analysis_server.dart

@@ -435,8 +435,9 @@ class AnalysisServer {
           channel.sendResponse(exception.response);
           return;
         } catch (exception, stackTrace) {
-          RequestError error =
-              new RequestError(RequestErrorCode.SERVER_ERROR, exception);
+          RequestError error = new RequestError(
+              RequestErrorCode.SERVER_ERROR,
+              exception.toString());
           if (stackTrace != null) {
             error.stackTrace = stackTrace.toString();
           }