Feature summary (what you would like to be able to do and where):
When editing the system message wmau-config.json, the types of values should be validated so that it fails errors out when invalid values are used prior to being saved, instead of failing after it is saved (and thus locking out all WMAU skin users on the wiki until it is fixed)

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
This would prevent fatal configuration issues from taking down the wiki, as seen in https://issue-tracker.miraheze.org/T13374

Benefits (why should this be implemented?):
It would prevent downtime since the misconfiguration wouldn't be saved :p