[WebAssembly] Improve feature validation error messages

Summary:
Add the names of the input files responsible for each error to the
messages.

Reviewers: sbc100, azakai

Subscribers: dschuff, jgravelle-google, aheejin, sunfish, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62704

llvm-svn: 362162