Fixed|DEH Reader: Crash parsing DEH patches referencing unknown definitions

DeHackEd patches are intended for a specific version of a game, which
we may not have default definitions for. In such a case, read into a
dummy definition so at least parsing *should* complete successfully.

Encountered this when regression testing Tango's MBF-compatible mod
"Mayan Mishap" (see: http://www.doomworld.com/vb/wads-mods/70845-mayan-mishap-final-release/)

(Although we don't presently support the MBF feature set, Doomsday
should (ideally) not crash when trying a mod that requires it).
DEH Reader: Standardized formatting of warning messages