Propagate deadStripOptimize()'s failure to the caller.

We want to make the program to exit with non-zero exit code if there's an error
during dead stripping.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@192771 91177308-0d34-0410-b5e6-96231b3b80d8
Fix a bug that the empty string could be added to dead strip root.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@192772 91177308-0d34-0410-b5e6-96231b3b80d8