[llvm-mca] Propagate fatal llvm-mca errors from library classes to driver.

Summary:
This patch introduces error handling to propagate the errors from llvm-mca library classes (or what will become library classes) up to the driver.  This patch also introduces an enum to make clearer the intention of the return value for Stage::execute.

This supports PR38101.

Reviewers: andreadb, courbet, RKSimon

Reviewed By: andreadb

Subscribers: llvm-commits, tschuett, gbedwell

Differential Revision: https://reviews.llvm.org/D50561

llvm-svn: 339594