wallet: Handle duplicate fileid exception

Summary:
Loading two identical wallet files caused a crash

Handle the duplicate fileid exception thrown at CheckUniqueFileid in tow cases:

• when duplicate wallets are set on the command line - catch in LoadWallets;
• when a duplicate wallet is loaded dynamically - catch in LoadWallet.

This is a backport of Core PR16923

Test Plan:
ninja all check-all

In bitcoin-qt, backup the currently open wallet to a file named wallet.dat,
then in the console window type loadwallet /path/to/wallet/.
Check that the program does not crash and you get a useful error message dialog.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8898