[DISCO-4084] Handle 204 as a successful empty response

[misaniwere]

[DISCO-4084]

The client was previously treating HTTP 204 as an error. 204 is a valid Merino API response indicating no suggestions are available for weather. getSuggestions() now returns Option<String> —> None for 204 and Some(json) for 200. The internal NoContent error variant has been removed.

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[gruberb]

Looks good. Some food for thought. I think it's easier to keep the codes on the top level.

[gruberb]

I would like to keep the error codes on the top level, for readability. The performance cost for unwrapping the .text() is very low here, and we could even do:

 match status {
      200 => Ok(Some(response)),
      204 => Ok(None),
      400 => Err(Error::BadRequest { code: status, message: response.text().to_string() }),
      422 => Err(Error::Validation { code: status, message: response.text().to_string() }),
      500..=599 => Err(Error::Server { code: status, message: response.text().to_string() }),
      _ => Err(Error::Unexpected { code: status, message: response.text().to_string() }),
  }