bgo#639531 - [filechooser] Handle the case where the user types a nonexistent_subfold...

bgo#639531 - [filechooser] Handle the case where the user types a nonexistent_subfolder/file.txt

The logic was to try to switch to that nonexistent folder and thus get
an error message presented.  However, no such message actually appears,
as the file chooser tries to switch to the closest parent folder that
actually exists, without bringing up an error message --- this is done
to cope with the case of the file chooser being started with a folder
that doesn't exist anymore.

Now, we just bring up an error message directly when we detect that
the user types a subfolder name that doesn't exist.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>