Better error messages for unknown combinators

Util/BlazeFromHtml.hs

@@ -119,16 +119,17 @@ fromHtml variant (Parent tag attrs inner) = case combinatorType variant tag of
     LeafCombinator -> [combinator]
 
     -- Unknown tag
-    UnknownCombinator -> error $ "Unknown tag: " ++ tag
+    UnknownCombinator -> error $ "Tag " ++ tag ++ " is illegal in "
+                                        ++ show variant
   where
     combinator = qualifiedSanitize "H." tag ++ attributes'
     attributes' = attrs >>= \(k, v) -> if k `elem` attributes variant
         then " ! " ++ qualifiedSanitize "A." k ++ " " ++ show v
-        else error $ "Unknown attribute: " ++ k
+        else error $ "Attribute " ++ k ++ " is illegal in " ++ show variant
 
     -- Qualifies a tag with the given qualifier if needed, and sanitizes it.
-    qualifiedSanitize qualifier tag =
-        (if isNameClash variant tag then qualifier else "") ++ sanitize tag
+    qualifiedSanitize qualifier tag' =
+        (if isNameClash variant tag' then qualifier else "") ++ sanitize tag'
 
     -- Check if we can drop the apply operator ($), for readability reasons.
     -- This would change: