Fix error message when api_key is absent

I just tried mkr for the first time, and the CLI displayed the following error. Yes, the reason was that the `MACKEREL_APIKEY` environment variable was absent on the machine.

```
$ mkr host status --host-id foo --status working
Error: undefined method `strip' for nil:NilClass
```

This PR improves the error message to suggest users what they should do next. If you don't prefer the error message, please let me know better one.