Fix confusing error message when lxml is not installed

warc-tiny

@@ -445,7 +445,7 @@ class ScrapeMode(ProcessMode):
 	def __init__(self, urlsOnly):
 		self._urlsOnly = urlsOnly
 
-		assert wpull is not None, 'Scrape mode requires wpull'
+		assert wpull is not None, 'Scrape mode requires wpull and lxml'
 		htmlParser = wpull.document.htmlparse.lxml_.HTMLParser()
 		elementWalker = wpull.scraper.html.ElementWalker()
 		scrapers = []