Maintenance mode support tweaks

[pedro]

Change copy/display to match what was in Core

[brandur]

Here's the current view for reference.

[markpundsack]

@brandur Didn't you have something better for explicit maintenance mode (vs just API unavailable)?

[markpundsack]

Or am I thinking of read-only mode (and is that different?)

[brandur]

I don't think we have either right now unfortunately :/ It requires some weird logic in Identity's error handlers.