Change schema registry to work without avro

[jaysonsantos]

These errors are being used just to export avro's exceptions but this means that you can never use schema-registry if you installed confluent kafka with confluent-kafka[schema-registry]

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[jaysonsantos]

[clabot:check]
it would be nice if you guys used https for the cla page

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[mhowlett]

yeah, this is bad. solution is not ideal, but resolves the problem and we can back it out without consequence if we want, eventually doing something better.

[mhowlett]

for some reason which i don't understand this is showing a conflict, and not allowing this to be resolved here, so I opened again in #1055 and merged that.