bugfix: make LoadWallet assigns status always

Summary:
Although loadwallet() in rpcwallet.cpp assumes LoadWallet() always
assign some value to the 'status', but LoadWallet() does not do so
in some situation.

This fixes above and prevends loadwallet() returns ambiguous error code.

This is a backport of core#19919

The problem is probably introduced in D10227 or D10229, but I wasn't able to reproduce
it.

Depends on D10231

Test Plan:

ninja
for i in $(seq 20); do test/functional/test_runner.py wallet_multiwallet; done

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10240