Asm Parser: support .error directive

The .error directive is similar to .err in that it will halt assembly if it is
evaluated for assembly.  However, it permits a user supplied message to be
rendered.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201999 91177308-0d34-0410-b5e6-96231b3b80d8