[REQ] [GO] Update an Error Message within client.decode to be Informative

Created by: confluent-alec-kendall

Is your feature request related to a problem? Please describe.

client.decode only supports decoding of Content-Types JSON and XML. If the Content-Type is not supported, an error message is produced.

The error message produced is vague. Instead, the error message should be informative.

Describe the solution you'd like

Update the error message from "undefined response type" to "Content-Type is not supported for decoding: " + contentType.

Describe alternatives you've considered

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Additional context

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