[yaml2obj] Improve error message when output file cannot be opened.

Patch by Sam Clegg!

Differential Revision:  https://reviews.llvm.org/D31351

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299546 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed