gio: handle GSimpleAsyncResult errors in _finish vmethods

gio: handle GSimpleAsyncResult errors in _finish vmethods

Originally, the standard idiom with GSimpleAsyncResult was to handle
all errors in the _finish wrapper function, so that vmethods only had
to deal with successful results. But this means that chaining up to a
parent _finish vmethod won't work correctly. Fix this by also checking
for errors in all the relevant vmethods. (We have to redundantly check
in both the vmethod and the wrapper to preserve compatibility.)

https://bugzilla.gnome.org/show_bug.cgi?id=667375
https://bugzilla.gnome.org/show_bug.cgi?id=661767