Merge #13564: [wallet] loadwallet shouldn't create new wallets.

Summary:
ea65182f03 [wallet] loadwallet shouldn't create new wallets. (John Newbery)

Pull request description:

A bug in the initial implementation of loadwallet meant that if the
arguement was a directory that didn't contain a wallet.dat file, a new
wallet would be created in that directory. Fix that so that if a
directory is passed in, it must contain a wallet.dat file.

Bug reported by promag (João Barbosa).

Tree-SHA512: 0a59fa8a33fde51a88544ad288b00e4995284fe16424f643076aaba42b8244fff362145217650ee53d518dfab7efbed4237632c34cdd3dcbbecaa9ecaab5fd7b

Backport of Core PR13564
https://github.com/bitcoin/bitcoin/pull/13564/

Depends on D4236

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4239