1. Use better error messages in LinkFiles().

1. Use better error messages in LinkFiles().
2. I think the caller of LinkFiles() should not ignore a true return value.
   (If you have a good reason why it ought to, feel free to revert
   this. It's just something that's been bugging me for a while.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@8760 91177308-0d34-0410-b5e6-96231b3b80d8