#17 Checkbot report misses some problematic pages.

[Paul Merchant, Jr.]

Checkbot omits certain pages from its error report under the following conditions

1. The page contains a broken URL found on other pages in the scan; and
2. The broken URL appears on a page scanned before the page in question; and
3. The broken URL is scanned (and thus determined to be broken) after the page in question has been scanned.

This can be fixed by the attached patch which modifies add_checked to add the current page to the list of parents of a broken URL that has already been checked.