Please add more descriptive error message for hostWhitelist property in environment.js

Hi I've no idea what that means. I struggled to understand where this hostWhitelist should be defined without success.

My currently dirty environment.js has this:

``` ruby

var ENV = {
modulePrefix: 'gitconfig-client',
environment: environment,
baseURL: '/',
locationType: 'auto',
hostWhitelist: ['localhost'],
EmberENV: {
hostWhitelist: ['localhost'],
FEATURES: {
hostWhitelist: ['localhost'],
// Here you can enable experimental features on an ember canary build
// e.g. 'with-controller': true
},
EXTEND_PROTOTYPES: {
hostWhitelist: ['localhost'],
Date: false,
}
},

APP: {
hostWhitelist: ['localhost'],
// Here you can pass flags/options to your application instance
// when it is created
}
};

```

And the erro I get when starting FastBoot with:

``` bash
ember fastboot --port 4200 --host localhost
```

Is this:

> 500 Unknown Error: Error: You are using Ember Data with no host defined in your adapter. This will attempt to use the host of the FastBoot request, which is not configured for the current host of this request. Please set the hostWhitelist property for in your environment.js. FastBoot Error: You must provide a hostWhitelist to retrieve the host