create wallet: don't allow entry of already existing wallet name

[MarnixCroes]

Is there an existing issue for this?

• I have searched the existing issues

Current behaviour

currently there is no check if the entered wallet name already exists.
it should check it and don't allow it/have a related message

as mentioned in #403 (review)

Expected behaviour

• not able to continue when entering an already existing wallet name.
• have a warning/error message that wallet name already exists.

Steps to reproduce

enter existing wallet name

Relevant log output

No response

How did you obtain Bitcoin Core

Compiled from source

What version of Bitcoin Core are you using?

a73577b

Operating system and version

linux

Machine specifications

No response

[jarolrod]

The Qt Widgets allows this, but will then fail to create the wallet, we do want to fail early, so agree with this