Picker should handle middleware errors

[sprohaska]

Picker currently ignores middleware errors. For example ,req.body will be empty if the body parser size limit is violated.

As a workaround, I've added a hack to pass errors via req.error to my fork: sprohaska@284f626

Ideally, Picker should support express-style error route handler that take an additional err.

[halvardos]

This is a serious problem as it stands, having req.body be empty and no end event fired because of a silent failure in the middleware is extremely hard to identify and debug.