Fixed #20278 -- ensured .get() exceptions do not recurse infinitely

A regression caused by d5b93d3281fe93cbef5de84a52 made .get() error
reporting recurse infinitely on certain rare conditions. Fixed this by
not trying to print the given lookup kwargs.
Fixed #20403 -- Ignore forms marked for deletion when validating max_num.
Fixed #20445 -- Raised original exception after command error
Tweaked unit test 'quick start' explanation.

Thanks Jeremy Dunck.
Fixed #20471 - Typos in topics/forms/formsets.txt
Fixed #9321 -- Deprecated hard-coding of help text in model ManyToManyField fields.

This is backward incompatible for custom form field/widgets that rely
on the hard-coded 'Hold down "Control", or "Command" on a Mac, to select
more than one.' sentence.

Application that use standard model form fields and widgets aren't
affected but need to start handling these help texts by themselves
before Django 1.8.

For more details, see the related release notes and deprecation timeline
sections added with this commit.
Add for_concrete_model to GenericForeignKey

Allows a GenericForeignKey to reference proxy models. The default for
for_concrete_model is True to keep backwards compatibility.

Fixes #17648.