Merge lp://staging/~bryce/launchpad/lp-796645-cron-dies-on-404 into lp://staging/launchpad

[r=sinzui][bug=796645] Log warning when bug trackers are not available.

The update-bugzilla-remote-components.py cron job can fail when getRemoteProductsAndComponents() hits various HTTPErrors on any bug tracker. (LP: #796645)

This merge adds a general exception handler to catch and ignore any errors thrown by the urllib2 call. This will allow data to be gathered from those bugzillas which are not broken. It does not attempt any corrections, retries, or workarounds for broken bug trackers, it just leaves any previously collected data in place and carries on.

Most of this branch is adding test code so we can synthetically test for HTTPErrors.

This merge modifies getRemoteProductsAndComponents() so that instead of creating a new scraper object per bugtracker, to instead allow the test code to pass in its own scraper, which is used for all requests instead of generating a scraper object for each URL.

The test code can then supply scrapers subclassed from BugzillaRemoteComponentScraper with hardcoded behaviors:

  StaticTextBugzillaRemoteComponentScraper replaces the static text
  parameter. Instead of hardcoding a check for this option, we allow the
  test code to provide the static text via the subclasses' getPage().

  FaultyBugzillaRemoteComponentScraper provides a way to simulate hitting
  various error codes by raising whatever errors we pass to it.

A assertGetRemoteProductsAndComponentsDoesNotAssert() helper method is added to allow us to run getRemoteProductsAndComponents() and check that no assertions were thrown.