Paginator.Page() should throw an exception for invalid [next|previous]_page_number()

[mehta.apurva@…]

The next_page_number() and previous_page_number() currently return an incremented or decremented number without verifying if the page is in the range or not. They should instead throw an exception.

Repro Steps
=======

from django.core.paginator import Paginator
objects = ['john', 'paul', 'george', 'ringo', 'bill', 'gates', 'steve', 'jobs']
p = Paginator(objects, 2)
p.count

8

p.num_pages

4

p.page_range

[1, 2, 3, 4]

p.page(1).has_previous()

False

p.page(4).has_next()

False

p.page(1).previous_page_number() ### Should throw Exception

0

p.page(4).next_page_number() ### Should throw Exception

5