dir: Propagate errors from copying summary{.sig}
dir: Propagate errors from copying summary{.sig}

This commit ensures that flatpak_remote_state_save_summary()
initializes the passed GError pointer when returning FALSE. I found this
when looking into https://github.com/flatpak/flatpak/issues/1255 because
at the time of that bug report flatpak_dir_update_appstream() had this
g_file_replace_contents() code in it, which would have caused a seg
fault in update_appstream() after an unsuccessful call to
flatpak_dir_update_appstream().

Fixes https://github.com/flatpak/flatpak/issues/1255

Closes: #1893
Approved by: alexlarsson

matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak