Zingo-CLI does not provide intuitive error message when wallet file dir is incorrect.

[idky137]

When launching Zingo-CLI from a wallet file the specifics of how the file should be presented are not documented and explicit error messages are not provided, specifically the path must be to the directory, not the wallet file.

LOG:

~/src/zingo-flow-tester exit status: 101
$ ll /home/idky137/.cache/zingo-cli/testnet_wallet/
total 1.8M
drwxr-xr-x 2 idky137 idky137 4.0K Mar 24 19:44 .
drwxr-xr-x 3 idky137 idky137 4.0K Mar  6 11:34 ..
-rw-r--r-- 1 idky137 idky137 1.8M Mar 24 19:43 zingo-wallet.dat
19:44:48 idky137 works main [ M M M M M D D D D D M M M ?? ?? ]
~/src/zingo-flow-tester
$ zingo-cli --chain testnet --server https://testnet.zec.rocks:443 --data-dir /home/idky137/.cache/zingo-cli/testnet_wallet/zingo-wallet.dat --waitsync balance
2026-03-24T19:44:53.131826Z ERROR zingolib::config: Couldn't create zcash directory!
File exists (os error 17)

thread 'main' panicked at zingolib/src/config.rs:513:21:
Couldn't create zcash directory!
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
19:44:53 idky137 works main [ M M M M M D D D D D M M M ?? ?? ]
~/src/zingo-flow-tester exit status: 101
$ zingo-cli --chain testnet --server https://testnet.zec.rocks:443 --data-dir /home/idky137/.cache/zingo-cli/testnet_wallet/ --waitsync balance
Launching sync task...
Launching save task...
[
    confirmed_orchard_balance: 53_208_463
    unconfirmed_orchard_balance: 0
    total_orchard_balance: 53_208_463

    confirmed_sapling_balance: 0
    unconfirmed_sapling_balance: 0
    total_sapling_balance: 0

    confirmed_transparent_balance: 0
    unconfirmed_transparent_balance: 0
    total_transparent_balance: 0
]
Save task shutdown successfully.
Zingo CLI quit successfully.