[llvm-mc] Better error handling in ENOENT case + test.

This is a follow up to r247518.
As a general note, I think we could do a much better job testing for
error conditions in tools. I already anticipated in a previous mail,
but while implementing this I noticed that the code coverage we have 
for error checking is pretty low. I can arbitrarily remove checks from 
several tools and the suite still passes.

Differential Revision:	 http://reviews.llvm.org/D12846

llvm-svn: 247582