[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

llvm-svn: 217873