Check for existing directory earlier

[AllenDowney]

As soon as the user enters the project name (or repo name? which is used for the directory name), check if the directory already exists and either quit or prompt for a new name.

Otherwise, the user has to answer all of the questions and THEN get an error message.

[pjbull]

Related issue from cookiecutter (open since 2016): cookiecutter/cookiecutter#821

Ideally this would be fixed and/or enabled upstream. There's basically no support for validation at all in cookiecutter's current architecture, so we couldn't fail fast without replacing substantially more of the package than we currently patch.

I'll add this to things we consider for v2, but I suspect it may land out-of-scope