Rollup merge of #69880 - RalfJung:miri-assert-error-sanity, r=oli-obk

Rollup merge of #69880 - RalfJung:miri-assert-error-sanity, r=oli-obk

miri engine: turn error sanity checks into assertions

We had these as debug assertions so far to make sure our test suite is clean, but really these are conditions that should never arise and also @eddyb told me to turn non-performance-critical debug assertions into full assertions so here we go. ;)

I propose that we do a check-only crater run to make sure this does not actually happen in practice.

r? @oli-obk