Update: as @Tgr pointed out in comment below, it is easier and safer to edit MediaWiki:noarticletext
page within MediaWiki software itself.
If you have filesystem access where your wiki is hosted, you can edit the noarticletext
key in [your wiki location in filesystem]/languages/i18n/en.json
(or your currently active language).
To make life easier you could have noarticletext
message include a {{No article text}}
template (as Wikipedia looks to have done) so that the message can later be edited from within MediaWiki by editing that template.
Googling uncovers a Special404 MediaWiki extension but it looks like that needs filesystem access as well, at least to first install it.