private String formatError(Schema schema, DocumentSummary summary, SummaryField field, String msg) {
        return "For " + schema + ", document-summary '" + summary.name()
               + "', summary field '" + field.getName() + "': " + msg;
    }
Iterates all summary fields with 'matched-elements-only' or 'select-elements-by' and validates that the field type is supported.

@author geirst
config-model/src/main/java/com/yahoo/schema/processing/SummaryElementsSelectorValidator.java