Prevent running yo chisel on an existing project

[luboskmetko]

@jakub300 wondering if we could prevent running yo chisel on an existing Chisel project? I know that yo asks about each override but I'm worried that less experienced users could accidentally override their work.

In case they try to run yo chisel on an existing Chisel project, something like A Chisel project already exists in this folder. Check out documentation for how to setup Chisel on an existing project could appear.