install: print directory name instead of file name if mkstemp fails

Printing the file name doesn't make sense since mkstemp failing means
that the file wasn't created.

Also add a test case for this.

Co-authored-by: Jose Luis Duran <jlduran@gmail.com>
Reviewed by: imp,jhb
Pull Request: https://github.com/freebsd/freebsd-src/pull/1383