fix: dereference error in UnmarshalJSON for BatchCall

[klaidliadon]

While testing some endpoints I found that I could not use errors.As with jsonrpc.Error consistently. After some debugging I found the cause: in some parts jsonrpc.Error is returned, while BatchCall uses *jsonrpc.Error.

This PR makes the type consistent.

This:

func handleRPCError(t *testing.T, err error) {
	if jsonErr := (&jsonrpc.Error{}); errors.As(err, jsonErr) || errors.As(err, &jsonErr) {
		if jsonErr.Code == -32000 {
			t.Skipf("skipping: %s", err)
		}
	}
}

Becomes:

func handleRPCError(t *testing.T, err error) {
	if jsonErr := (&jsonrpc.Error{}); errors.As(err, jsonErr) {
		if jsonErr.Code == -32000 {
			t.Skipf("skipping: %s", err)
		}
	}
}

[VojtechVitek]

Nice catch, thank you :) ✅