Instead of spamming stderr with g_warning, use the new
emit_error method of the GtkStyleProviderPrivate interface
to emit an error if loading an image fails.
We should not hardcode a scale of 1, this leads to
pixellated upscaled images at scale=2, even if the source
is an svg. By passing the proper scale, we can load the
svg at the correct size.
If we fail to load the image for a -gtk-recolor() expression,
fall back to using the image-missing icon instead of crashing,
and include more details in the warning message.
