InstrProf: Give coverage its own errors instead of piggy backing on instrprof

Since the coverage mapping reader and the instrprof reader were
emitting a shared set of error codes, the error messages you'd get
back from llvm-cov were ambiguous about what was actually wrong. Add
another error category to fix this.

I've also improved the wording on a couple of the instrprof errors,
for consistency.

llvm-svn: 236665