replace syslog with warns

replace syslog with warns

As discussed with Chris Nighswonger on #koha, this patch
removes the calls to syslog and replaces them with warns
so that error messages generated by the labels code
are sent to the Apache error log.  This avoids splitting
this sort of logging across multiple files and is consistent
with current practice in most of the rest of Koha.

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>