Adds an onFailure callback to delegate error handling to the app

[jasonmit]

This adds an onFailure hook that can be passed into the fastboot middleware.

The use case is apps may already have a way of handling errors. This allows someone to inject their own handler logic, which could be as simple as next(err) to allow their apps error handler to take over.

[tomdale]

Rather than introducing a new callback, I think (in the interests of Express ecosystem consistency) we should consider copying the fallthrough API from express.static().

[jasonmit]

Sounds great, what should I default to?

[tomdale]

static() defaults to true, but I think that probably gets used standalone less than FastBoot. false would preserve our current semantics, which IMO are correct, but I'm open to hearing from others who disagree.

[arjansingh]

we can probably close this now that #11 is merged.

[jasonmit]

I believe what I'm after is slightly different, I want to delegate error handling entirely to the caller.

In #11 it looks like it res.send with html in some cases which might conflict with whatever the error handler is doing.