Handle missing issuer claim for GitHub JWT

# Problem

If we're lacking the proper ENV variables for getting an integrations token, we'll get the following for code that tries to exchange our application credentials for a token:

```
(MatchError) no match of right hand side value: {:error, %CodeCorps.GitHub.APIError{documentation_url: "https://developer.github.com/v3", errors: nil, message: "Missing 'issuer' claim ('iss') in assertion", status_code: 401}}
```

We should handle this error more directly by explaining what's happening here (i.e. you're missing some configuration that makes JWT work).