icon theme: Ensure to propagate an error
icon theme: Ensure to propagate an error

If the svg pixbuf loader is not available, we end up with criticals
from gtk_css_image_icon_theme_draw because gtk_icon_info_load_symbolic
returns NULL without setting an error.

Avoid this by propagating the load error.

alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk