Fix several new errors caught by mypy 0.501.

Clear out a bunch of easy to review errors, so we can focus on the more
complicated ones.

tools/documentation_crawler/documentation_crawler/spiders/common/spiders.py

@@ -97,3 +97,4 @@ class BaseDocumentationSpider(scrapy.Spider):
             self._set_error_state()
         else:
             raise Exception(failure.value)
+        return None