[raw_fd_ostream] report actual error in error messages

Summary:
Previously, we would emit error messages like "IO failure on output
stream". This change causes use to include information about what
actually went wrong, e.g. "No space left on device".

Reviewers: sunfish, rnk

Reviewed By: rnk

Subscribers: mehdi_amini, llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D39203

llvm-svn: 316404