Tags: pablomarx/NewTen

Add an Integrity Check to the ROM downloader.

When it first reads the manufacturer and hardware type bits,
if they're unknown values it'll stop and display a dialog with
an error message and the option to stop or continue.

This is to deal with a hypothetical situation where a Newton
has a bug in it's Serial Debugger that treats read memory
as erase memory, and has EEPROM.  This check would limit
the damage to at most the two words, instead of erasing
the entire EEPROM.