Propagate error in LazyEmittingLayer::removeModule.

Summary:
Besides being the better thing to do, not doing so will triggers an assert with LLVM_ENABLE_ABI_BREAKING_CHECKS.

Reviewers: lhames

Reviewed By: lhames

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36700

llvm-svn: 310906