[mips] Improve the error messages given by MipsAsmParser.

Summary: Changed error messages to be more informative and to resemble other clang/llvm error messages (first letter is lower case, no ending punctuation) and updated corresponding tests.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D5065

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217873 91177308-0d34-0410-b5e6-96231b3b80d8
Interpreter: Hack around a series of bugs in MSVC 2012 that copies around this
move-only struct.

I feel terrible now, but at least it's shielded away from proper compilers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217875 91177308-0d34-0410-b5e6-96231b3b80d8