I've decided to remove `MISSING_MODEL_MESSAGE`. The only reason to introduce this attribute was that `ForeignKey` messages started with "The field has relation with..." while m2m message started with "The field has *many to many* relation with...". This attribute was a negligible detail that a contributor needed to track through class hierarchy. This was done in  https://github.com/chrismedrela/django/commit/1ed2c52e886ea556f4b8c0ec57d2193994b42ad6.