Test for cfg dir existance

When testing on win8.1 and win7 sp1, I found that the error is different, and tbh, it looks to me like we intended to only choose a dir _if_ it exists - so I've added the existance test

also - if we're falling back to a last hope (``cwd()``) then use that dir, don't 'hope' that there is a .boot2docker dir in it.