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.
