Add better error handling

[miltondp]

Currently, we are not handling errors. For instance, if your .env file selects an Ollama model and the model has not been pulled, the error will appear in the logs, but the UI will replace the selected text with an empty string. We could add some error handling in the coordinator agent, but it would be better to have a more general, global error handling strategy that returns a JSON object for the UI to show to the user.