MC: formalise some assertions into proper errors

Now that clang can be used as an assembler via the IAS, invalid assembler inputs
would cause the assertions to trigger.  Although we cannot recover from the
errors here, nor provide caret diagnostics, attempt to handle them slightly more
gracefully by reporting a fatal error.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209387 91177308-0d34-0410-b5e6-96231b3b80d8