newusers: Add missing error handling

newusers: Add missing error handling

Some errors were being reported in stderr, but then they weren't really
being treated as errors.

If mkdir(2) for EEXIST, it's possible that the sysadmin pre-created the
user dir; don't fail.  However, let's keep a log line, for having some
notice that it happened.

Also, run chmod(2) if mkdir(2) failed for EEXIST (so transform the
'else if' into an 'if').

Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>