Need to indicate if message produce schema serialization issue happend on key vs value schema

[jlrobins]

When produce / serialization error happens, the exception returned to extension side needs to indicate if the issue encountered with the key schema or the value schema. Please include some machine-readable clear attribute like "messagePart: [key|value]" or whatnot in the exception JSON.

Knowing which will help extension indicate to the user, say, through only marking either the key or value subset of the produced message as having error.