avroregistry: better error messages

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.