Introduce a Safe Mode when there was a problem with configuration

[Saviq]

We could also use the diagnostic feature to (for a time?) signify a problem to whoever's at the display.

That I like (for Frame). That could be generalised to any startup/initialization error reporting: we would just need to reexec with a "safe" config and error text.

Originally posted by @AlanGriffiths in #3628 (comment)

[Saviq]

When there's a problem with Mir starting up, we should fall back to a safe mode, ignoring the invalid configuration, but still start up.

Additionally, showing an overlay informing the user about the configuration issues should be possible, with the compositor implementation deciding how (choices could include overlay, notification, shell integration, logging etc.).