FileCheck: Return parse error w/ Error & Expected

Summary:
Make use of Error and Expected to bubble up diagnostics and force
checking of errors in the callers.

Reviewers: jhenderson, jdenny, probinson, arichardson

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63125

llvm-svn: 363900