TableGen: Add some more helpful error messages
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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@325795 91177308-0d34-0410-b5e6-96231b3b80d8

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