feat: improve error handling in the TUI

[hugodutka]

Addresses #40 and #24.

• Removes this.error from the ChatManager to avoid bugs with clearing the error and replaces it with an onError handler, which prints the error to the console.
• Formats errors to be less red.
• Adds "Chat Error" and "Agent Log/Error" prefixes to the corresponding logs to make it easier to distinguish them in the UI.