rpc: more accurate checking of callback/subpub method signatures

[stephenfire]

The first argument of a Pub/Sub function should not be a pointer to context.Context, otherwise it will be registered as a subscription and its corresponding callback.hasCtx==false. This results in the parameter not having the correct value when called.

The problem description of the old code is as follows:

1. Suppose we define a subscription method, the first parameter is *context.Context.
2. When executing serviceRegistry.registerName for service registration, the isContextType method returns true, and isPubSub returns true, making callback.isSubscribe==true.
3. In the makeArgTypes method, because the first parameter Type!=contextType, the previously created callback.hasCtx==false.
4. In the callback.Call method, because callback.hasCtx==false, the correct context.Context cannot be passed into the corresponding method.

[stephenfire]

Make sure callback/subscription returns error instead of *error.

If a callback or subscription method returns *error, it will still be judged as callback or subscription, and the position of the returned *error will be recorded in callback.errPos. In callback.Call, when the corresponding position value of callback.errPos is not nil, executing err := results[c.errPos].Interface().(error) will cause panic of "interface conversion: *error is not error: missing method Error"

[fjl]

Thanks!