Rare PrimProc exception not caught correctly

In PrimProc's command.cpp a bad command would throw an exception. This wouldn't happen in the wild but happened whilst I was adding new commands.

If the exception is thrown it is caught in the threadpool instead of the command processor. This means it is silently handled instead of an error fed back to ExeMgr. ExeMgr then waits forever.

There could be other instances of this happening too. General improvements to exception handling in PrimProc's command processor would solve this.