Improve client robustness to simple messages

[cnweaver]

Description

As a user, I do not want the client to abort if it finds any message without an envelope (without even letting me see the offending message).

Currently, the client demands that all messages have a JSON envelope with a "format" field specifying the data format and a "content" field contain the actual data. This is fine, but since we give users direct access to Kafka, it only takes one person writing a message (with kafkacat or any other tool which does not enforce the hop client's notions of formatting) which does not conform to this to make the client perpetually unhappy, potentially making it impossible for users to read the any subsequent messages in the affected topic if their start point for reading includes the 'bad' message.

Definition of Done

The hop client should make it possible to do some combination of the following:

• continue reading even after encountering an unintelligible message
• work with JSON messages which are not otherwise wrapped
• display offending messages which do not meet formatting expectations

Tasks/Items:

• TBD