bug: Check for existing wallet before importing/creating wallets

[theborakompanioni]

Expected behavior

A user will be warned early if a wallet name is already taken.

Actual behavior

On the create or import page, if a duplicate wallet name is given, the user is not warned and will error only later in the process (e.g. after providing a mnemonic phrase).

Steps to reproduce the problem

1. Imit the regtest environment
2. Try to create or import a wallet names "Satoshi" (which already exists)
3. Notice that an error occurs when submitting the last step - which could be known earlier

Thanks to @editwentyone for reporting this.

[theborakompanioni]

Resolved in #720. Thank you @zmjohnso 🙏