Provide a better error message when trying to use repo-backed media with self-hosted Tina

[scottgallant]

Describe the issue that you're seeing. Any Loom videos or screenshots usually help a lot!

Repo-stored media isn't currently supported when self-hosting. Another provider is required (cloudinary, s3, digital ocean) or the user must manually add content to the repo (via git). Currently, the user only sees a generic error message when trying to set this up and it leads to confusion. See here

https://discord.com/channels/835168149439643678/1199452734248734783/1225355498732261428

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