[TableGen] Make an error message slightly more informative

Reviewers: ab, spop, stoklund

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D19192

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266823 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed