add_custom_command: Format files in error message in a single line

23 jobs for add-custom-target-byproduct-checks
Status Name Job ID Coverage
  External
passed cdash

passed cmake-dash3win10-windows-release-ninja

00:16:21

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:37:47

passed cmake-minoc-linux-release-lint-iwyu-tidy

00:01:44

passed cmake-moonshade-osx-debug-ninja

00:09:45

passed cmake-moonshade-osx-release-makefiles

00:10:42

passed cmake-terfin-windows-release-vs2019

00:34:14

passed ghostflow-check-master

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager