gis-page: Add an error argument to save_data()
This allows errors to be returned from pages if saving data fails.
Currently, the top-level handling for these is to print a g_warning()
,
but that could change in future to display the errors in the UI.
This introduces one behaviour difference from before:
gis_driver_save_data()
now stops saving data after the first error
from a page, rather than carrying on calling save_data()
on all pages
regardless.
Signed-off-by: Philip Withnall withnall@endlessm.com
Fixes: #95 (closed)