Add assertions for empty selector results

[spulec]

One common scraper pattern is

    results = doc.select("table p > a")
    for result in results:
        ...

Unfortunately, if the webpage changes and doc.select returns an empty list we have no way of knowing that the scraper is now broken. The correct solution is probably to go back and change to something like

    results = doc.select("table p > a")
    if not results:
        raise AssertionError("No report links found for %s" % url)
    for result in results:
        ...

[divergentdave]

Closed by #210