[scudo][standalone] Add error reports

Summary:
This change adds fatal error messages for various error conditions that
will be added later in the code.
This also addresses a `TODO` now that we have `reportCheckFailed` (which
lead me to notice a few variables that were not cased properly so I
changed that as well).

Reviewers: morehouse, hctim, vitalybuka

Reviewed By: morehouse, hctim, vitalybuka

Subscribers: mgorny, delcypher, jfb, jdoerfert, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D59551

llvm-svn: 356556