empty pathnames, zero cylinders, etc. It doesn't seem fair to allow
people to write out bochsrcs that they cannot read back in without
patching them up by hand!
- in harddrv.cc add the equivalent checks with BX_PANICs so that at least
we don't start simulating with incomplete device configuration.
- make a few error messages more clear
- modified: main.cc iodev/harddrv.cc