Instead of just cryptically exiting 0 when opening a non-existant
repodata or stagedata, with the only message existing in the debug
output, show an error to the user on stderr.
mentioned in #519
If repodir is given as a relative path, it will not work because it will
try to open the dir relative to itself because it already chdir()ed into
repodir. This fixes xbps-rindex -r when relative paths are specified.
fixes#519