Improve error handling

[Ithamar]

At the moment, any error when loading a model (or when switching modes) is not shown, and the operation will simply fail, potentially leaving the UI in an indeterministic state (e.g. mode-specific features like export missing, etc).

I should build a little console UI that shows the error to the user, and have an easy way of copying the relevant error information so it can be used to create tickets either here on github or on the CMM Discord server.