[scudo] Add verbose failures in place of CHECK(0)

Summary:
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).

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: filcab, mgorny, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D48199

llvm-svn: 334843