@Override
    public boolean equals(Object o) {
        if (!super.equals(o)) return false;

        ErrorMessage other = (ErrorMessage) o;
        if (this.source != null) {
            if (!this.source.equals(other.source)) return false;
        } else {
            if (other.source != null) return false;
        }

        return true;
    }
Returns the source producing this error, or null if no source is specified
container-search/src/main/java/com/yahoo/search/result/ErrorMessage.java