`fatxpool`: add reason for `Invalid` status.

[michalkucharczyk]

1. The error was just "Invalid", but it feels like the error back across the websocket should have had the information about lack of funds

We could improve this and add some reason for Invalid variant here. Looks like the reason of Invalid was never exposed from the txpool API.

Originally posted by @michalkucharczyk in #8479

[haanhvu]

I'm working on this