Make BlockContentForm::save() return

There is at least one existing issue about this where I argued that the documentation is pretty weird as this is a form callback and form callbacks don't have return values.

I see. Unless I am misreading things, the interface stays that it is supposed to return one of the two constants used in the merge request. So, this suggests there is a deeper architectural issue at play here.

Seems like this still needs work and agreement on best path forward.

@berdir, are you saying that EntityFormInterface::save() and EntityForm::save() should not return? Or are you saying that those should return but BlockContentForm::save() should not?

I note that the tests still pass with the return statement added.

Closing as duplicate of 🐛 Remove return value from EntityFormInterface::save Needs work , see discussion there.