Have all allocate statements pass out "stat" values that are then checked

[nusbaume]

Whenever an allocate statement is used, it should be made to pass out a stat value, that is then checked for success conditions. If the allocate was un-successful, then endrun should be called, with the associated message containing the stat value.

Although this might require modifications across CAMDEN, it will make model failures related to allocations much easier to identify and debug, which should help both users and developers.