More helpful error when allowed redirect domains are defined (#104)

Make the error message when a forbidden destination is given to
`/redirect-to`, to help users better understand what allowed usage looks
like. Motivated by #103.

Before: 

```
$ curl http://localhost:8080/redirect-to?url=https://google.com
Forbidden redirect URL. Be careful with this link.
```

After:

```
$ curl http://localhost:8080/redirect-to?url=https://google.com
Forbidden redirect URL. Please be careful with this link.

Allowed redirect destinations:
- example.com
- example.org
```