validator: fix indent
validator: do not raise Exception but yield error
validator: change tow class methods to static methods
validator: extract _load_document method
validator: simplified exception handling
validator: extract _validate_all that chains all validation
crawler: check basic validity in _mine

_mine checks now if the 'type' attribute is available and does not fail
anymore with an exception.
crawler: do not distinguish between valid/invalid URLs

We now save all visited URLs in one set. The invalid URLs could be found
as the keys of the _errors dictionary.
crawler: yield errors as they appear

We want to display the errors during the crawl and not only at the
end. To the crawler has to be a generator that yields the errors.
cli: simple display for the errors