Detect if the a worktree root has not been defined

[mminns]

If the root worktree folder is not yet configured then an attempt to create a new worktree can result in checkoug out the branch within the current folder.

git wta open --branch issue/XOS
Create a new worktree with new branch issue/XOS at issue/XOS? [y/N]:

Its should detect the missing root folder config and at least warn the user.