Clearer explanation when exception has no message

"No exception supplied" is misleading; actually there is an exception,
but there's no message string.
Fixed #20242 - Added a regression test for prefetch_related.

Issue was fixed by removal of chunked reads from
QuerySet iteration in 70679243d1786e03557c28929f9762a119e3ac14.

Thanks Simeon Visser for the patch.
Fixed #20484 -- Added model validation for GenericIPAddressField

GenericIPAddressField must not allow blank for NOT NULL fields

Thanks Erik Romijn.
Fixed #20525 -- Added versionadded for clearsessions.

Thanks wiml@.
Fixed #20250 - Added a regression test for negated Q + annotate

Thanks nott.
Fixed #15653 - Error in admin pagination tag.

Thanks jcumbo@ for the report and adamzap
and nott for the patch.
Fixed #15697 -- Made sqlindexes aware of auto-created tables

Thanks mbertheau for the report and Ash Christopher for the
initial patch.
Fixed #20331 -- Allowed admin actions to serve StreamingHttpResponses

Thanks Edwin.
Fixed a failure on Python 3 introduced by 5939864616d56cf949f0bca348a8e05230b62fe0.

Refs #15653.
Don't hard-code class names when calling static methods

normalize_email should be called on the instance, not the class. This
has the same effect normally but is more helpful to subclassers. When
methods are called directly on the class, subclasses can't override
them.
Fixed regroup example.

Chicago was missing.
Made an app-cache test not dependent on all models in defer_regress
Fixed #16436 -- defer + annotate + select_related crash

Correctly calculate the ``aggregate_start`` offset from loaded fields,
if any are deferred, instead of ``self.query.select`` which includes all
fields on the model.

Also made some PEP 8 fixes.
Fixed #20509 - Proper parsing for dumpdata --pks option.

Thanks weipin for the report and Baptiste Mispelon for the patch.
Fixed #20272 - Moved update_fields existence check into Model._do_update.

Thanks Gavin Wahl.
Fixed #16856 - Added a way to clear select_related.

Thanks Carl for the suggestion and David Cramer for the patch.
Merge pull request #1027 from intgr/debug_no_exc_message

Clearer explanation when exception has no message
Fixed #19425 - Added InlineModelAdmin.get_extra hook.

Thanks dave@ for the suggestion and Rohan Jain for the patch.
Added missing stacklevel arg in forms/models.py
Fixed #20502 -- Flushed i18n caches for a defaultfilter test

Thanks Chris Wilson for the report and initial patch.