Define error-handling middleware functions explicitly (without arity detection)

I propose adding an explicit `app.error` method for defining error-handling middleware functions:

``` js
app.error(function(err, req, res, next) {
res.status(500).send('Something broke!');
});
```

Instead of:

``` js
app.use(function(err, req, res, next) {
res.status(500).send('Something broke!');
});
```

It's so easy to forget the `next` argument when it's not being used in the body of the function, and that changes the whole meaning of the middleware. `express` is one of the only packages that has this pattern. Others that used this pattern in the past (i.e. `superagent`) have since removed it.

Furthermore, this clashes with popular linting rules like ESLint's [`no-unused-vars` rule](http://eslint.org/docs/2.0.0/rules/no-unused-vars) which enforce that all named arguments must be used in the function body. Users who see this rule and remove the un-unsed `next` parameter will be unwittingly changing the behavior of their program.

No one expects removing an unused parameter to change the behavior of a program.

``` js
app.use(function(err, req, res, next /* <-- unused, guess I'll remove this... */) {
res.status(500).send('Something broke!');
});
```

The four argument middleware convention should be deprecated in favor of `app.error`, but support for it could remain for a long time, or even indefinitely. I just want to be able to recommend that folks use `app.error` going forward.