[BUG] Slient invalid document link resolution

[thorin-schiffer]

Describe the bug
When a document has a invalid link, fetch_links=True results in a silent non-resolution to the Link object. This is a hardly debuggable case if a collection of objects is fetched.

To Reproduce
So if attribute requires a domain

class Domain(Document):
    ...

class Attribute(Document):
    ...
    domain: Link[Domain]
    ...

and the linked domain was removed, requesting a collection like await Attribute.all(fetch_links=True).to_list() will have both resolved and unresolved links to domain, which is hard to distinguish from await Attribute.all().to_list() without fetched links.

Expected behavior
I would prefer an exception, but alternatively another type can be set, like UnresolvableLink, or None or Link instance with some flag.

[roman-right]

Hi! Thank you for the issue. I'll think about the interface for this