java.lang.Error thrown during mass indexing are swallowed and not reported to the user

See https://discourse.hibernate.org/t/massindexer-not-working-on-latest-snapshot/3872/10?u=yrodiere

In org.hibernate.search.mapper.orm.massindexing.impl.FailureHandledRunnable#run in particular, we only catch and report RuntimeException, but we completely ignore Error. We should report the error some way.