Low: libcrmservice: use NAGIOS_STATE_UNKNOWN for internal nagios errors

Previously, servicesgeneric_error() (and thus serviceshandle_exec_error())
used PCMK_OCF_UNKNOWN_ERROR as the rc for internal errors for nagios actions.

However, that maps to NAGIOS_STATE_WARNING, when NAGIOS_STATE_UNKNOWN (meaning
that the check could not be executed) is more appropriate.