Improve the performance of node.preAllocateCheck()

The node.preAllocateCheck() function gets run a lot during the scheduling cycle. The function is effectively a boolean function, either the node can be allocated on or it cannot. Instead of returning a bool it returns nil for true and a full error message for false. The thing is, that error message is never read anywhere. From what I can tell, the only thing the code does is check if there is an error message but not the contents. By removing the error message code, it should improve performance.