fatal: destination path 'substrate-ui' already exists and is not an empty directory.

[ltfschoen]

If a user runs the command substrate-ui-new substrate then it may have created a subdirectory named substrate-ui with contents, whether it finished cloning or not. If the user runs the same command again and the subdirectory exists and has some contents then it will give error fatal: destination path 'substrate-ui' already exists and is not an empty directory.

Refer to links where this issue was raised:

• Forum post: https://forum.parity.io/t/substrate-documentation-ideas-and-feedback/166/5
• Riot chat: https://matrix.to/#/!uBLZhuoBfbnDWESzNJ:matrix.parity.io/$154522026977353LYsOf:matrix.parity.io?via=matrix.parity.io&via=matrix.org