Return the current status of a model.

Model status. Possible values are:

XPRD_RT_OK	Normal termination
XPRD_RT_ERROR	An error occured during execution
XPRD_RT_MATHERR	Mathematical error ( e.g. division by zero)
XPRD_RT_I/OERR	Input/output error ( e.g. cannot open file)
XPRD_RT_STOP	Bit set if execution has been interrupted
XPRD_RT_FDCLOSED	Connection to the remote host has been lost
XPRD_RT_RUNNING	Model currently running

When the status is XPRD_RT_FDCLOSED, the model is no longer usable and the only possible operation is XPRDunloadmod or XPRDdisconnect that must be called in order to release local resources used by the model.