TableGen: Add some more helpful error messages

Summary: Some fairly simple changes to start with.

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43552

Change-Id: I0c92731b36d309c6edfcae42595ae1a70cc051c9
llvm-svn: 325795