Support plain text errors

Hi,

I have a spec where the error message is of string type and the mime type is plaintext:

/some/path:
  post:
    responses:
      '200': ...
      '500':
        description: Server error
        content:
          text/plain; charset=utf-8:
            schema:
              type: string

This gets generated into a method returning Result<ResponseValue<_>, Error<ByteStream>>. The problem happens when deserialize the response:

match response.status().as_u16() {
    200u16 => ResponseValue::from_response(response).await,
    500u16 => Err(Error::ErrorResponse(ResponseValue::stream(response))),
    _ => Err(Error::UnexpectedResponse(response)),
}

The from_response method has a bound to fmt::Debug on the error type, which is not implemented for ByteStream, thus failing compilation.

I worked around it by simply removing the bound, but I'm wondering if there's a better solution. I can also open a PR to fix it if this workaround is good.

Start at progenitor-client/src/progenitor_client.rs, especially from_response and its fmt::Debug bound, then reproduce generation from the YAML example in the issue. Done means the generated client compiles for a text/plain string error response and handles that response without requiring an unsupported ByteStream bound.