Galley polysemy (4/5) - Error effects

[pcapriotti]

This PR is about introducing Error effects in Galley, as part of the polysemy refactoring, and towards fixing https://wearezeta.atlassian.net/browse/FS-40.

The all-encompassing Wai.Error has now been split into several different error types, corresponding to some arbitrary error categories. The mapping between error categories and swagger error documentation is not yet established, and Servant integration is still missing.

Eventually, I would like to make the CanThrow combinator in Servant type-safe, by removing the most error effects from the list of errors that are accepted by the final Sem monad (AllErrorEffects in Galley.API.Error), and allowing use of CanThrow to "opt-in" to certain errors. Internal errors would still be handled by default.

This PR is still not addressing the main point of https://wearezeta.atlassian.net/browse/FS-40. That will be taken care of once the refactoring is finished.

Overall plan for the Polysemy refactoring of Galley

1. Turn Galley into a newtype over Sem, and introduce "access" effects.
2. Add "store" effects and convert Data code into interpretations.
3. Turn placeholders into actual effects.
4. Add fine-grained Error effects. (this pr)
5. Make Galley into a type synonym over Sem, and get rid of Galley0.

Checklist

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