post fmt3: properly error out on unknown chars in math mode
post fmt3: properly error out on unknown chars in math mode

The lexer had a nasty habit of just dumping unknown chars to stdout.  This
of course didn't accomplish much.

Signed-off-by: Josef 'Jeff' Sipek <a620f141433c70aa9e43778305a6c68cbfadfb25@josefsipek.net>

jeffpc/blahgd,jeffpc/blahgd