[ELF] - Remove ScriptLexer::Error field and check ErrorCount instead.

D35945 introduces change when there is useless to check Error flag
in few places, but ErrorCount must be checked instead.

But then we probably can just check ErrorCount always. That should simplify
things. Patch do that.

Differential revision: https://reviews.llvm.org/D36266

llvm-svn: 310046