Create the directory DIR and optionally any nonexistent parent dirs.
If DIR already exists as a directory, signal an error, unless
PARENTS is non-nil.
Interactively, the default choice of directory to create is the
current buffer's default directory. That is useful when you have
visited a file in a nonexistent directory.
Noninteractively, the second (optional) argument PARENTS, if
non-nil, says whether to create parent directories that don't
exist. Interactively, this happens by default.
If creating the directory or directories fail, an error will be raised.