Clarify error codes to be used

[dosse]

In https://github.com/sdmx-twg/sdmx-json/blob/master/structure-message/docs/1-sdmx-json-field-guide.md#error, https://github.com/sdmx-twg/sdmx-json/blob/master/data-message/docs/1-sdmx-json-field-guide.md#error and https://github.com/sdmx-twg/sdmx-json/blob/master/metadata-message/docs/1-sdmx-json-field-guide.md#statusmessage, the documentation still says: "code - Number. Provides a code number for the error message. Code numbers are defined in the SDMX 2.1 Web Services Guidelines."

These "SDMX 2.1 Web Services Guidelines" defining specific SDMX error codes have been replaced by the new SDMX REST API 2.0.0 where these specific SDMX codes have been dropped.