[clang-doc] Use llvm::createStringError and canonicalize error messages

"Bad block found.\n" -> "bad block found"

The lower cased form with no full stop or newline is more common in LLVM
tools.

Reviewed By: juliehockett

Differential Revision: https://reviews.llvm.org/D66783

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@370155 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed