koji-search-containers: raise when metadata.json download fails
    
    Prior to this change, if the Koji packages web server returned an HTTP
    error for the container build metadata.json file, we would not catch
    this error. The program might crash later on when parsing
    response.json(), or elsewhere.
    
    Raise immediately if we get an HTTP error when downloading the
    metadata.json file. This makes it easier for developers to understand
    the root cause of a failure in this code.