Fixed #29403 -- Made PyLibMCCache backend handle TooBig exception from pylibmc.

[hramezani]

Ticket: https://code.djangoproject.com/ticket/29403
Continue to #9811

I removed the self._cache.delete(key) and it seems we don't need it. if I add a test in PyLibMCCacheTests like:

    def test_large_value(self):
        cache.set('key', 'a')
        large_value = 'a' * (1024 * 1024)
        cache.set('key', large_value)
        value = cache.get('key')
        self.assertIsNone(value)

It will be passed successfully. Do we need to add this test?

[felixxm]

Thanks for your efforts! However I'm going to close this per comment.