Fixed cli restore in existing wallet folder

[kwek20]

Description of change

Checks before restoring a stronghold file if there is an existing stronghold.
The check for restoring when accounts exist was already in place, with error: "can't restore backup when there are already accounts"
Also adds a check for snapshot_path being a file, not folder (it crashes oddly if you send a folder path)

Links to any relevant issues

Closes #1068

Type of change

• Bug fix (a non-breaking change which fixes an issue)

How the change has been tested

Ran the following scenarios:

• Import an export in empty folder
• Import an export in a folder with wallet, but no account
• Import an export in a folder with wallet with accounts
• Import from the same folder as export (this errors saying already has an account, or succeeds if there wasnt any)

Change checklist

Tick the boxes that are relevant to your changes, and delete any items that are not.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes

[thibault-martinez]

Changelog