Based on user reports at Project:Support_Desk, many of our more inexperienced users get confused by our error for missing extension.json.
Given that making a typo here is pretty easy, its likely this is one of the first mediawiki error a new sysadmin will likely see, we should make it friendly so they have a good early impression of mediawiki.
Change 845096 had a related patch set uploaded (by Brian Wolff; author: Brian Wolff):
[mediawiki/core@master] registration: Add a friendly error message for missing extension
Change 899596 had a related patch set uploaded (by Jdlrobson; author: Brian Wolff):
[mediawiki/core@REL1_40] registration: Add a friendly error message for missing extension
Change 845096 merged by jenkins-bot:
[mediawiki/core@master] registration: Add a friendly error message for missing extension
Change 899596 merged by jenkins-bot:
[mediawiki/core@REL1_40] registration: Add a friendly error message for missing extension