Clearer explanation when exception has no message

"No exception supplied" is misleading; actually there is an exception,
but there's no message string.
Fixed #20443 -- Import errors in settings cause confusing error messages
Replaced `and...or...` constructs with PEP 308 conditional expressions.
Merge pull request #1217 from erikr/better-settings-errors2

Fixed #20443 -- Import errors in settings cause confusing error messages
Patch for ticket #20506 - documentation type
Merge pull request #1224 from yishaibeeri/master

Fixed #20506 - Typo in transactions docs.
Fixed #20507 -- SubqueryConstraint alias relabeling

The SubqueryConstraint defined relabeled_clone(), but that was never
called. Instead there is now clone() and relabel_aliases() methods for
SubqueryConstraint.

A related problem was that SubqueryConstraint didn't correctly use
quote_name_unless_alias() of the outer query. This resulted in failures
when running under PostgreSQL.
Fixed #20508 -- ReST typo.
Fixed URL for six inventory.
Fix #20505: Typo in BinaryField documentation.
Reimplemented PostGIS spatial_version with cached_property
Fixed #12491 -- Tweak checkbox widget help text alignment on the admin.

Thanks master' for the report and Niels van Dijk for the fix
Fixed #20507 -- SubqueryConstraint alias handling

MySQL should work now, too.
Fixed #17582 - Added message to DoesNotExist exceptions.

Thanks simon@ for the suggestion and JordanPowell
for the initial patch.
Merge pull request #1226 from bmispelon/ticket-20505

Fixed #20505 -- Typo in BinaryField documentation.
Fixed #16137 - Removed kwargs requirement for QuerySet.get_or_create

Thanks wilfred@, poirier, and charettes for work
on the patch.
Fixed #20501 -- failure of datetime queries with timezones under Oracle
Fixed #20013 -- A test for sqlall fails under Oracle
Fixed #11603 - Added django.test.SimpleTestCase.assertFormsetError

Thank-you Martin Green for the patch.
Fixed #20182 - admin lookup should treat 0 as False for __isnull

Thanks Benjie Chen.