obj2yaml: Improve error reporting

Previously only the error codes were reported which
meant that useful information about malformed inputs
was not shown.

Differential Revision: https://reviews.llvm.org/D34008

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305609 91177308-0d34-0410-b5e6-96231b3b80d8