Consolidate all errors, use onErrorHtml

[pbrisbin]

Prior to this commit, some errors would be thrown (missing parameter, invalid state, incorrect approot) while others would be handled via the set-message-redirect approach (handshake failure, fetch-token failure, etc).

This commit consolidates all of these cases into a single DispatchError type, and then uses MonadError (concretely ExceptT) to capture them all and handle them in one place ourselves.

It then updates that handling to:

• Use onErrorHtml

  onErrorHtml will, by default, set-message-redirect. That make this behavior neutral for users running defaults. For users that have customized this, it will be an improvement that all our error cases now respect it.

• Provided a JSON representation of errors

• Attach a random correlation identifier

The last two were just nice-to-haves that were cheap to add once the code was in this state.

Note that the use of MonadError requires a potentially "bad" orphan MonadUnliftIO instance for ExceptT, but I'd like to see that instance become a reality and think it needs some real-world experimentation to get there, so here I am.

[pbrisbin]

First commit is the thing, with a minimized diff. Subsequent diffs are refactor.