With the improved escape sequence parser, it becomes easy to also improve

With the improved escape sequence parser, it becomes easy to also improve
diagnostics.  Distinguish "incomplete escape sequence", "invalid special
character", and "unknown special character" from the generic "invalid
escape sequence", also promoting them from WARNING to ERROR because
incomplete escape sequences are severe syntax violations and because
encountering an invalid or unknown special character makes it likely
that part of the document content intended by the authors gets lost.