Improve handling apps without migrations while running migrate command.

[Andriy Sokolovskiy]

Consider situation - ​https://pypi.python.org/pypi/django-contrib-comments/1.5 does not have a migration inside.
It depends on Content-Type model.
So we get something like:

claudep: the traceback shows that the exception happens when syncing apps without migrations
[4:23pm] knbk: that "worked" with a big disclaimer on 1.7 but was unsupported, but somehow breaks on 1.8
[4:23pm] claudep: so the issue is "a unmigrated app depends on an app with migrations"

The error is django.db.utils.ProgrammingError: relation "django_content_type" does not exist.
Steps to reproduce - simply add django_comments to clean project and run python manage.py migrate. (But, I can reproduce with on postgres, but not on sqlite for example).

We need to improve handling apps, that does not have a migration, and show more user-friendly error rather than throwing exception, which user must investigate.

[Claude Paroz]

Raising a meaningful error telling which app failed (maybe which SQL statement?) would be welcome.

[Andriy Sokolovskiy]

I will work in patch

[Daniel Blasco]

+1 This caused headaches because of the cryptic exception.

A way to get around it for now is in settings to comment out the apps without migrations folder. Then migrate, uncomment the conflicting apps and finally migrate again.

[Tim Graham]

The situation has likely changed on master (1.9) now that migrations are compulsory for all apps.

[Claude Paroz]

I guess that boat has sailed now.