[scudo][standalone] Add error reports
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