code: private String constructErrorMessage(ProcessingReport report) {
          StringBuilder stringBuilder = new StringBuilder();
          stringBuilder.append("Json validation failed: ");
          report.forEach(processingMessage -> stringBuilder.append(processingMessage.getMessage()));
          return stringBuilder.toString();
      }
docs: Constructs the error message of a failed validation based on the processing report passed from
  com.github.fge.jsonschema.core.report
  @param report The report containing the error message
  @return A string representation of all messages contained in the report