* tsdb/errors.MultiError: implement Unwrap
the multierror was hiding some errors in Mimir. I also added unit tests because I had them handy from a similar change I and yuri did in XXX and some time ago
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Signed-off-by: Dimitar Dimitrov <dimitar.dimitrov@grafana.com>
Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>