fix suggestion

The plugin returns OK status even there are no connection to the snmp target host. For example I had error (with OK status):
ERROR: No response from remote host’10.201.11.124′

So what I suggest is to add exit 2 status in sub SNMP_getvalue:

if(!defined($res)){
print “ERROR: “.$snmp_session->error.”n”;
exit 2;
}