TableGen: add some more helpful error messages

Summary: Change-Id: I6f3dacf675a4126134577616e259696bebdade3a

Reviewers: tra, simon_tatham, craig.topper, MartinO, arsenm

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D47429

Change-Id: I614de12a4c154c6d53c090f2f3e53ad2d09942c5
llvm-svn: 333436