avroregistry: better error messages

[rogpeppe]

The Confluent Avro registry produces very uninformative error messages.

For example, when trying to upload an incompatible schema to a subject, the returned error says nothing about why the schema is incompatible

We could do better than that. For example, we could obtain the latest version of the schema from that subject and include that in the error message or (better) do some kind of a diff and say exactly how it's incompatible.