Rethink how sync adapter exposes batch errors / failed records.

[thomcc]

At the moment it's largely taken straight from desktop, but this seems error prone and less than ideal.

See #12 (comment) for some more context.

┆Issue is synchronized with this Jira Story

[mhammond]

I think the tl;dr of this is that posting a batch either works or it doesn't. Allowing some records to fail but have the batch succeed seems too vague to be useful without much tighter semantics (which can be added later if they actually appear in practice)

[jdragojevic]

Closing - sounds like not something we committed to doing.