[ALL-561] feat(frontend|backend): Display error messages in the chat

[amanape]

End-user friendly description of the problem this fixes or functionality that this introduces
When something wrong occurred in the agent or backend, the frontend would be empty and hardly display any indicator to the user.

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions
Utilize send_error to send ErrorObservation's as well and modify the frontend to catch and display them. Some examples:

Exception caught by report_error

Error sent directly by send_error or as API error response are toast

TODO

• Errors from send_error should be toast
• Otherwise, inline in chat interface
• Ensure that inline chat messages are the inline-style after a reload (they may be converted to user messages)

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Link of any specific issues this addresses

[tofarr]

LGTM, though I would like to test some of the different error states in the morning.