api! Remove the experimental (and incomplete) ERROR_NUMBERS code The purpose of this feature is to optionally prepend standardized numbers to error and warning messages. The ERROR_NUMBERS feature was first drafted in libpng-1.2.0 and further developed in libpng-1.4.0; and yet, it was always disabled by default, and never completed. In the light of a recent report concerning the security hazards arising from enabling this code in custom libpng builds, we think it's best to simply remove all the code written to support this feature. Based on removal in libpng18, but keeping functions and options as no-ops for backwards compatibility. Co-authored-by: Cosmin Truta <ctruta@gmail.com>