Don't hide errors when creating manpage

Don't hide errors when creating manpage

Apparently the default behaviour for pod2man changed at some point and
now it exits with a syntax error instead of exiting successfully with a
<POD ERRORS> section.

Now we do display the syntax issue:
[ruben@localhost collectd]$ make -j
  POD2MAN  src/collectd.conf.5
src/collectd.conf.pod around line 10361: You forgot a '=back' before '=head2'
POD document had syntax errors at /usr/bin/pod2man line 69.
make: *** [Makefile:9641: src/collectd.conf.5] Error 255