Improve error handling from Express

A small change that I think improves the error handling... the scenario I have is the following:
- an error occurs during the logic to generate the request to the Identity provider (error the implementation of a custom `cacheProvider`)
- with the existing code, this is essentially a silent error, and Passport-SAML redirects to the `failureRedirect` option, but there is no opportunity for any higher level application code to see the error, log it, etc...

Changing this line to call `error()` seems to do the right thing, which let's an Express error handling middleware catch the error if configured globally for example after all routes.  Example:
```
app.use(function(err, req, res, next) {

    // log and handle err, which can come from Passport-SAML catastrophic error...
});
```

It seems like calling `error()` is the right thing according to Passport comments:
```
      /**
       * Internal error while performing authentication.
       *
       * Strategies should call this function when an internal error occurs
       * during the process of performing authentication; for example, if the
       * user directory is not available.
       *
       * @param {Error} err
       * @api public
       */
```