simulation: better error for "monitor port used"

[jeffallen]

If another student is using an onet-based simulation on deterlab, then you'll get this error because they are "squatting" on port 10000 on users.deterlab.net:

F : (              platform.(*Deterlab).Start: 332) - ssh port forwarding exited in failure: exit status 255

The error message needs to indicate that the probable fix for this is to add "-mport 10001" in order to use a different port from the other user.

Much better would just be for the monitor port to be chosen automatically by the system, don't know if that is easy enough to do.