Refactor lib errors

[bvotteler]

We want the returned errors from the API to be easier to parse than just having a (hard-coded) string.
It would also be more useful if the error itself could the API entry point to make it easier for the UI to see which call caused an error.

This might need to be split into multiple tasks and tackled as a milestone.
It will be a massive breaking change, so treat with care.