Teach Lit to catch OSError exceptions when creating a process during the
execution of a shell command. This can happen for example if the
``RUN:`` line calls a python script which can work correctly under
Linux/OSX but will not work under Windows. A more useful error message
is now shown rather than an unhelpful backtrace.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220227 91177308-0d34-0410-b5e6-96231b3b80d8