[lit] Change to raise InternalShellError for all command execution issues.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174101 91177308-0d34-0410-b5e6-96231b3b80d8
[lit] Add a test for internal shell execution behaviors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174102 91177308-0d34-0410-b5e6-96231b3b80d8