rpc: improve error codes for internal server errors (#25678)

This changes the error code returned by the RPC server in certain situations:

- handler panic: code -32603
- result marshaling error: code -32603
- attempt to subscribe via HTTP: code -32001

In all of the above cases, the server previously returned the default error
code -32000.

Co-authored-by: Nicholas Zhao <nicholas.zhao@gmail.com>
Co-authored-by: Felix Lange <fjl@twurst.com>

rpc/handler.go

@@ -48,7 +48,6 @@ import (
 //    if err := op.wait(...); err != nil {
 //        h.removeRequestOp(op) // timeout, etc.
 //    }
-//
 type handler struct {
 	reg            *serviceRegistry
 	unsubscribeCb  *callback
@@ -354,7 +353,10 @@ func (h *handler) handleCall(cp *callProc, msg *jsonrpcMessage) *jsonrpcMessage
 // handleSubscribe processes *_subscribe method calls.
 func (h *handler) handleSubscribe(cp *callProc, msg *jsonrpcMessage) *jsonrpcMessage {
 	if !h.allowSubscribe {
-		return msg.errorResponse(ErrNotificationsUnsupported)
+		return msg.errorResponse(&internalServerError{
+			code:    errcodeNotificationsUnsupported,
+			message: ErrNotificationsUnsupported.Error(),
+		})
 	}
 
 	// Subscription method name is first argument.