Url._url_check should return None

There's no point in returning True if it's never checked and won't ever
be False.
Implicitly returning None or raising an exception is more idiomatic.

waybackpy/wrapper.py

@@ -64,7 +64,6 @@ class Url:
         """Check for common URL problems."""
         if "." not in self.url:
             raise URLError("'%s' is not a vaild URL." % self.url)
-        return True
 
     def _clean_url(self):
         """Fix the URL, if possible."""