Print an error message instead of an assertion failure.
Print an error message instead of an assertion failure.

This assertion is failing on a Chromium builder and I cannot figure out why.
This patch let it print out more info.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@297353 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld