[feature] API error handling

[tsmethurst]

This PR reworks almost all of the client and s2s api endpoints to provide better more consistent and informative error handling, as well as a few other little fixups.

Main changes:

1. Allow serving html errors if that's the client's preference -- very nice for 404 statuses etc, and closes [security] Make sure 404s from invisible statuses and 404s from bad routes are identical #58
2. Debug log non-200 codes passed through the new error handler in a consistent way; error log internal server errors.
3. Make a few remaining processor functions return gtserror.WithCode for more granular error handling.
4. Make the 404 page more informative for users so they don't get puzzled about why they can't see certain things.
5. Redirect html requests to AP user URIs to that user's profile or the relevant status instead.
6. Lots of other little tidies up.