Add error argument to save_data() vfunc
Currently, several of the
save_data() implementations can fail, and they either do so silently or emitting a
g_warning(). The function calling them has no idea about the failure, and continues to call the next page’s
save_data(), which will probably end up causing a cascade of failures.
GError argument to them and stop on the first error.
Discussed briefly in !72 (comment 708585)