Result of the form `(body, mime_type)`, or `ValueError`.

def distributions_impl(self, tag, run):
    """Result of the form `(body, mime_type)`, or `ValueError`."""
    (histograms, mime_type) = self._histograms_plugin.histograms_impl(
        tag, run, downsample_to=self.SAMPLE_SIZE)
    return ([self._compress(histogram) for histogram in histograms],
            mime_type)