ensureDir() did not handle one last error case and there was some logic
in the main() function that belonged to ensureDir() as well. It was also
creating a directory with a hardcoded 0700 mode, regardless of what mode
was passed to it.
This refactors it a little to fix the broken behavior, avoid redundant
checks by taking advantage of the behavior of MkdirAll, and move the
extra logic from main() into ensureDir().
a0b7ac402d