starting to hack in fetch failed (including OP_FETCH_FAILED)

starting to hack in fetch failed (including OP_FETCH_FAILED)
going really badly as far as code-readability and clean design is concerned
but is progressing

a truly dreadful hack: OP_TRAP works (LDST Exceptions) because the
main decoder (PowerDecoder2) is used by core for the Trap pipeline.

unnnnfortunately... for MMU, a *Satellite* decoder (PowerDecodeSubset)
is used.  and Satellite decoders *only* understand *instructions*.
(which they part-decode locally).

therefore a manual override of the satellite decoder insn_type
and fn_unit is required when OP_FETCH_FAILED occurs.

truly awful.