IGN:Fix regression that prevented error being raised when no username and password are specified in the GUI for a recipe that needs them

resources/recipes/economist.recipe

@@ -18,7 +18,6 @@ class Economist(BasicNewsRecipe):
     __author__ = "Kovid Goyal"
     description = 'Global news and current affairs from a European perspective'
     oldest_article = 7.0
-    needs_subscription = False # Strange but true
     INDEX = 'http://www.economist.com/printedition'
     cover_url = 'http://www.economist.com/images/covers/currentcovereu_large.jpg'
     remove_tags = [dict(name=['script', 'noscript', 'title'])]

src/calibre/web/feeds/news.py

@@ -501,8 +501,10 @@ class BasicNewsRecipe(Recipe):
             if isinstance(self.feeds, basestring):
                 self.feeds = [self.feeds]
 
-        if self.needs_subscription and (self.username is None or self.password is None):
-            raise ValueError('The %s recipe needs a username and password.'%self.title)
+        if self.needs_subscription and (\
+                self.username is None or self.password is None or \
+                (not self.username and not self.password)):
+            raise ValueError(_('The "%s" recipe needs a username and password.')%self.title)
 
         self.browser = self.get_browser()
         self.image_map, self.image_counter = {}, 1