Werk #15421: Improve agent updater logging

Previously, the agent updater reported many errors coming from the agent bakery with this unhelpful log output:

Unexpected answer from Checkmk server: Missing json data.
Maybe we are talking to an agent bakery from before Checkmk 2.0?

This happened for all errors that can not be interpreted by the agent bakery in a meaningful way.

Starting with this Werk, the agent updater will instead propagate the unknown error, e.g.:

Agent Bakery: Unhandled exception: KeyError: 'Signatures'
See ~/var/log/web.log on site 'mysite' for details.

As a last resort, if the agent updater still receives a response that it can't parse, it will still log Unexpected answer from Checkmk server: Missing json data., but this time without the note about an old agent bakery, as this was not helpful at all.

Bonus:
The error from the above example is now handled correctly:

The deployment server provides an agent but that is not signed.

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