static void problemReading(Exception e) {
    LOG.log(Level.WARNING, "problem reading zipkin_dependencies", e);
  }
Returns true when the zipkin_dependencies table exists and has data in it, implying the spark job
has been run.
zipkin-storage/mysql-v1/src/main/java/zipkin2/storage/mysql/v1/HasPreAggregatedDependencies.java