[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