document.open (and file.newFromTemplate) with a path relative to the
working directory took the SVG's folder from Path::parent(), which is
"" for a bare file name and was then dropped, so relatively linked
images were "not found" (and, linked by their bare name, not listed in
missingLinks either). A relative path is now made absolute first: its
folder is the working directory, as for the absolute path.
vectorcraft-cli convert printed only the export's warnings; what reading
the input left out now comes as importWarnings.
Fixes#1604
13bd24ba79