Checkmk Werk 15421: Improve agent updater logging

ID: 15421 Title: Improve agent updater logging Component: agents Level: 1 Class: Bug fix Version: 2.3.0b1 Previously, the agent updater reported many errors coming from the agent bakery with this unhelpful log output: C+: Unexpected answer from Checkmk server: Missing json data. Maybe we are talking to an agent bakery from before Checkmk 2.0? C-: 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.: C+: Agent Bakery: Unhandled exception: KeyError: 'Signatures' See ~/var/log/web.log on site 'mysite' for details. C-: As a last resort, if the agent updater still receives a response that it can't parse, it will still log <tt>Unexpected answer from Checkmk server: Missing json data.</tt>, but this time without the note about an old agent bakery, as this was not helpful at all. <i>Bonus</i>:<br> The error from the above example is now handled correctly: C+: Agent Bakery: Missing signatures in status response from central bakery. Maybe agents are unsigned? C-: