Clearer explanation when exception has no message

"No exception supplied" is misleading; actually there is an exception,
but there's no message string.
Fixed random aggregation_regress test_more_more_more() failure

The cause was assuming that an unordered queryset returns the values
always in the same order.
Fixed #20513 - Expanded docs on QuerySet caching.

Thanks seddonym.
Fixed #20228 - Documented unique_for_date and exclude behavior.

Thanks Deepak Thukral for the patch.
Fixed #20523 - Incorrect form field for FilePathField.

Thanks sane4ka.sh@ for the report.
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.
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