crest.prepare could use an empty directory

[polyluxus]

currently crest.prepare creates a directory if it can and exits if it finds one

let the script check if the dir is empty, if so, then use it, if not we still can fail

[polyluxus]

This isn't that straight forward in bash.

Workaround:

• Try to remove the empty dir.
  • If it succeeds: Well done!
  • if not: die with error