standarize django's check messages levels usage

[skarzi]

In #91 we are introducing more Django checks and according to plans this number of checks will grown, so it's time to establish some rules of check messages levels and it's usage.

Django checks messages has following levels (they are similar to logging levels):

• DEBUG
• INFO
• SUCCESS
• WARNING
• ERROR

Reference: https://docs.djangoproject.com/en/3.0/topics/checks/#messages

[sobolevn]

My idea is that:

• We should issue WARNING messages when there's something not quite right with the app (we found any kind of rule violations)
• We should issue ERROR messages when our app cannot operate properly

This way user can control their CI level with check --fail-level= command.
By default our warnings would not fail the build.

[sobolevn]

Related: https://github.com/wemake-services/django-test-migrations/pull/91/files/a7f72b086d3f0076418674febc745d4626fdb5b5..4b6eeb44aa23441d3179ad3bcce76a4845078afa#r457584209