[1.5.X] Added WizardView.file_storage exception message and docs

Thanks Danilo Bargen for the patch.

Backport of af7ea808d8 from master

docs/ref/contrib/formtools/form-wizard.txt

@@ -493,6 +493,21 @@ Advanced ``WizardView`` methods
             context = self.get_context_data(form=form, **kwargs)
             return self.render_to_response(context)
 
+.. method:: WizardView.get_cleaned_data_for_step(step)
+
+    This method returns the cleaned data for a given ``step``. Before returning
+    the cleaned data, the stored values are revalidated through the form. If
+    the data doesn't validate, ``None`` will be returned.
+
+.. method:: WizardView.get_all_cleaned_data()
+
+    This method returns a merged dictionary of all form steps' ``cleaned_data``
+    dictionaries. If a step contains a ``FormSet``, the key will be prefixed
+    with ``formset-`` and contain a list of the formset's ``cleaned_data``
+    dictionaries. Note that if two or more steps have a field with the same
+    name, the value for that field from the latest step will overwrite the
+    value from any earlier steps.
+
 Providing initial data for the forms
 ====================================
 
@@ -534,6 +549,16 @@ This storage will temporarily store the uploaded files for the wizard. The
 :attr:`file_storage` attribute should be a
 :class:`~django.core.files.storage.Storage` subclass.
 
+Django provides a built-in storage class (see :ref:`the built-in filesystem
+storage class <builtin-fs-storage>`)::
+
+    from django.conf import settings
+    from django.core.files.storage import FileSystemStorage
+
+    class CustomWizardView(WizardView):
+        ...
+        file_storage = FileSystemStorage(location=os.path.join(settings.MEDIA_ROOT, 'photos'))
+
 .. warning::
 
     Please remember to take care of removing old files as the