Should kmd fail more gracefully or offer options when files already exist?

[kwinkunks]

Currently the pip cache and the target environment folder cannot already exist. When flailing around, this can result in some uncaught exceptions and a lot of typing rm this and that.

Pip cache: if cached files/folders exist then line 101 in fetch.py fails with FileExistsError: [Errno 17] File exists. Probably we should use the cache if it's there (it's a cache after all), or if it must not exist then maybe we should just delete it at the end of the build?

Target environment folder: if this already exists then line 54 in cli.py gives: mv: cannot move ‘my-rel-0.0.1/private/mtha/dev/geocomp-release/prefix/my-rel-0.0.1/root’ to ‘my-rel-0.0.1/root’: File exists.

Maybe kmd should check in advance if the target exists already and throw a more informative error, and/or we could have an option to clobber existing — or even use the current shell's noclobber option (off by default in bash, i.e. clobbering is allowed).

[kwinkunks]

Propose dealing with this along with #326