rpc: change no wallet loaded message to be clearer

Summary:
Changes the no wallet is loaded rpc error message to be clearer that no
wallet is loaded and how the user can load or create a wallet. Also
changes the error code from METHOD_NOT_FOUND to RPC_WALLET_NOT_FOUND as
that makes more sense.

This is a backport of core#20101

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10437