Is error handling middleware necessary?

[creynders]

As suggested by @mattapperson in #15 it might be useful to have error handling middleware. Decisions to make: How are they registered? To qualified hook identifiers? Or action identifiers? Or post only? What would their signature be? (err, p0, p1, p2) or simply (err)? Do we allow async error handlers or strictly synced? How does this integrate with promises? Also, what sequence would they run in? Before the final callback or after?