Show error when a block media field uses a field name without crop

[haringsrob]

Summary

So this happened from experience, you define a block (maybe your first time) but you do not have the crop in the config.

When the block form is validated, we could add this check and then notify the user (in debug mode): Hey you have this field name but no attached formatted.

OR, and this might be better,

we provide a "default" crop config that is always used in case no other can be found.

[ifox]

Providing a default free crop (as in no specific ratio, using null) would make sense to me, since it simply allows cropping and doesn't crop the attached image by default. I think this would make sense as a global default for missing crops on models, blocks and settings.