configfile.c: Abort if any config file could not be read.

configfile.c: Abort if any config file could not be read.

So far, when including all files from some directory, errors while reading
any of those config files have been reported but otherwise ignored. So,
collectd would run with some potentially incomplete configuration which is
not what I would expect.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>