fire an event when a path fails to load

[ssube]

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The loaders do not give any information when a path fails to load, which can lead to the interface hanging, or an uncaught error being printed over the game output. Wrap validation errors and catch any others, ensuring the every load-path event is followed by a success or error event.

This should use some friendly formatting for Ajv errors, which can potentially be localized. Validation errors while loading config should use the same formatting.