private Severity getMaxSeverity(final ReferenceDataResult result) {
        return Severity.getMaxSeverity(
                NullSafe.list(result.getMessages())
                        .stream()
                        .map(LazyMessage::getSeverity)
                        .collect(Collectors.toList()),
                Severity.INFO);
    }
Log a lookup failing to return a value.  Whether it actually logs depends on things like
whether there were any errors, state of ignoreWarnings, state of trace, etc.
stroom-pipeline/src/main/java/stroom/pipeline/xsltfunctions/AbstractLookup.java