fix wrong error message: pool-max-size may not be null

In jboss-cli I get:
/deployment=perftest-1.0.0.war/subsystem=ejb3/stateless-session-bean=PerfCall:write-attribute(name=pool-max-size,value=30)
{
    "outcome" => "failed",
    "failure-description" => "JBAS014746: pool-max-size may not be null",
    "rolled-back" => true
}
After this patch I get:
{
    "outcome" => "failed",
    "failure-description" => "JBAS014749: Operation handler failed: JBAS014338: Not implemented yet",
    "rolled-back" => true
}
which seems to be the right thing.