Thanks jdunck for the suggestion.

Fixed #21015 -- Fixed MigrationLoader when importlib.import_module returns a file module or an empty directory.

The documentation and comments now all use 'an' to
refer to the word SQL and not 'a'.

1. Moved common logic between assertContains and assertNotContains
   into a separate function _assert_contains()

2. Moved common logic between assertTemplateUsed and
   assertTemplateNotUsed into a separate function
   _assert_templateused()

Thanks Marc Tamlyn for the report and review.

Removed "makemigrations --force" from docs since it doesn't actually exist.

Thanks Keryn Knight for the suggestion.

Thanks tuxcanfly.

refs [fb1dd6b1] and [1c4a9bd9]

exc_value might be None even though there's an exception, at least on
Python 2.6. Thanks Thomas Chaumeny for the report.

Fixed #21034.

Forward-port of a8624b22 from 1.6.x.

Thanks marky1991.

Thanks Mathieu Leplatre for the report and initial patch.

Thanks jeroen.pulles at redslider.net for the report.

Refs #20998.

Thanks Mathieu Leplatre for the report and the initial patch.

Refs #1707 and 1c0c879b.