Special exit code when migrations are needed.

[peb-adr]

As mentioned shortly in #155 (comment), it would be nice to have the manage-tool exit on a seperate code (rather than just 1) for every subcommand that couldn't succeed because migrations in the backend are missing.

This would come in handy e.g. during an update, where manage commands might be called before the migrations have finished (or started even).
If the exit code would indicate this case, we could react to that better.

At first I was thinking 2, but that's conventionally reserved as well.
So I think we could introduce 3 here as our first custom exit code.

[normanjaeckel]

Does the backend tell me whether migrations are required? How?

[normanjaeckel]

I still do not know how to guess whether migrations are required (except for the initial-data command where the backend recently implemented some stuff).