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

[ldionne]

Summary

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.

llvm/cmake/modules/AddLLVM.cmake

def relpath(p):\n
     if not p: return ''\n
     if os.path.splitdrive(p)[0] != os.path.splitdrive(base)[0]: return p\n
     if haslink(p) or haslink(base): return p\n
     return os.path.relpath(p, base)\n
 if len(sys.argv) < 3: sys.exit(0)\n
 sys.stdout.write(';'.join(relpath(p) for p in sys.argv[2].split(';')))"
     ${basedir}
     ${pathlist_escaped}
-    OUTPUT_VARIABLE pathlist_relative)
+    OUTPUT_VARIABLE pathlist_relative
+    ERROR_VARIABLE error
+    RESULT_VARIABLE result)
+  if (NOT result EQUAL 0)
+    message(FATAL_ERROR "make_paths_relative() failed due to error '${result}', with stderr\n${error}")
+  endif()
   set(${out_pathlist} "${pathlist_relative}" PARENT_SCOPE)
 endfunction()
 
 # Converts a file that's relative to the current python file to an absolute
 # path. Since this uses __file__, it has to be emitted into python files that
 # use it and can't be in a lit module. Use with make_paths_relative().
 string(CONCAT LLVM_LIT_PATH_FUNCTION
   "# Allow generated file to be relocatable.\n"