[lit] Remove uses of deprecated except syntax.

 - Since we only have a few of these, use the cumbersome method of getting the
   exception object from 'sys' to retain the current pre-2.6 compatibility.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187854 91177308-0d34-0410-b5e6-96231b3b80d8