The current FailureHandler mechanism was fairly opaque with regard to the
failure reason due to using CHECK(0). Scudo is a bit different from the other
Sanitizers as it prefers to avoid spurious processing in its failure path. So
we just dieWithMessage using a somewhat explicit string.
Adapted the tests for the new strings.
While this takes care of the OnBadRequest & OnOOM failures, the next step
is probably to migrate the other Scudo failures in the same failes (header
corruption, invalid state and so on).
Maybe also define it out of line? No use making the class definition harder to read if we don't even want to inline this function.