Custom mapfile import doesn't notify the user on error, and is also hard to discover

[ExpHP]

No error message

When importing a custom mapfile, if an error occurs, simply nothing happens as an error is printed to JSON console. Need a user facing error. A toast?

Discoverability

• Zero says he didn't check the gear because he assumed it would probably just have a Light/Dark toggle.
• We can add a line to the */ins and */var pages that says

Using mapfiles from truth (change in settings)