Nicer error messages when a repository in a link doesn't exist

Currently, if a repository in a link doesn't exist (likely because the repository was deleted), an error such as the one below is raised:

```
Couldn't search issues on repository foo/bar: {\"message\":\"Validation Failed\",\"errors\":[{\"message\":\"The listed users and repositories cannot be searched either because the resources do not exist or you do not have
permission to view them.\",\"resource\":\"Search\",\"field\":\"q\",\"code\":\"invalid\"}],\"documentation_url\":\"https://developer.github.com/v3/search/\"}
```

Ideally, a better-sounding and nicer error that actually explains what's happening would be returned.