Community Wishlist Survey 2023/Allow custom error messages in the event of outages

Proposed by Dimpizzy

Proposal summary: Allow wiki admins to set up a custom error message that appears when the wiki is down due to an outage or maintenance. Allow the message to be set beforehand and automatically become visible, or during an outage provide a mechanism to set the message, or both.

Full proposal: I think that in the event of a long term outage like in December 2022, it would be nice for wikis to be able to provide some wiki specific information on the error page, such as possible alternatives to the wiki that the user can use in the meantime, or information on where the wiki admin can be reached outside of Miraheze. Maybe there could be a setting somewhere in the wiki where the error message can be set beforehand. Although, I think the better option would be a way to set/change the message after the wiki is already down, as admins might forget/not know about the option beforehand. Either just as a verbal request on e.g. Discord to Miraheze admins, or a way to enter text somewhere like meta.miraheze.org if it's not down as well and automatically change the error message.