rpc: Add specific error code for "wallet already loaded"
Summary:
Add a separate RPC error code for "wallet already loaded" to avoid having to match on message to detect this.
Backport of core#20964.
Test Plan:
ninja check-functional
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D17953