Check salt before user enters password

When you enter a too long (?) salt, an error is thrown:

```
$ openssl rand -hex 32
5f9eb64892c210857de415686a1ae73d2a78ff4fce25543626aafe2195a56381

$ argon2-cli 5f9eb64892c210857de415686a1ae73d2a78ff4fce25543626aafe2195a56381
Enter password: adsadsasd
Error: "Invalid salt: invalid Base64 length"
```

This check should happen before the user is prompted for a password.