fix: add error level handling to ErrorMessage class and constructor

[janole]

This pull request introduces a more structured error handling system by adding severity levels to error messages.

✨ New Features

• Error messages now have a level property, which can be set to "debug", "warn", or "error".
• Debug-level error messages are now hidden from the UI by default and will only be shown when the application is in debug mode.

⚙️ Under the Hood

• The ErrorMessage class in src/ai/custom-messages.ts has been updated to include the new level property.
• The error handling logic in src/ai/work.ts and src/ai/session/session.ts has been updated to use the new structured error messages, setting the level to "error".
• The ErrorMessageView component in src/ui/messages/error-message-view.tsx now conditionally renders messages based on their level and the application's debug state.