[llvm-lit] Improve the error message when make_paths_relative() fails

Previously, if make_paths_relative() failed due to some reason, it would
happily keep going and set the ${out_pathlist} to the standard output
of the command, which would be the empty string if the command failed.

This can lead to issues that are difficult to diagnose, since the calling
code will usually try to keep going with a variable that was set to the
empty string.

Differential Revision: https://reviews.llvm.org/D89985