Make error handling consistent.
Make error handling consistent.

Some functions in Writer reports error using HasError, and some reports
their return values. This patch makes them to consistently use HasError.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@265156 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld