a11y: Add proper error reporting to value collection

a11y: Add proper error reporting to value collection

We're currently overloading NULL to mean both "this value is undefined,
and should be reset to its default" and "the value collection failed".
Let's do error reporting right, by using GError to mean "the collection
failed, for this specific reason"; then, we can use a NULL return value
to signal that the accessible attribute should be reset to its default
value.

This is only relevant for pointer-sized attribute values: strings,
references, and reference lists; numeric, boolean, tristate, and token
values either cannot be undefined, or have a specific "undefined" value.