Complete demoting assumption failures

[agroce]

Making abandoned tests (due to assumptions, almost always) ERROR level is too noisy for fuzzing, where we have to guess and check, not solve. Makes brute force and libfuzzer in particular irritatingly verbose without careful log level control.

No real harm in demoting, and if these are worrisome in symex, we should just resolve that in symex front ends or maybe do a check on if we're symbolic.