Further improve CSRF error messages

	* Recommend checking if the HTTP client is persisting cookies

yesod-core/Yesod/Core/Handler.hs

@@ -1576,11 +1576,18 @@ data CSRFExpectation = CSRFHeader Text (Maybe Text) -- Key/Value
 
 csrfErrorMessage :: [CSRFExpectation]
                   -> Text -- ^ Error message
-csrfErrorMessage expectedLocations = "A valid CSRF token wasn't present. Because the request could have been forged, it's been rejected altogether. Check the Yesod.Core.Handler docs of the yesod-core package for details on CSRF protection.\nTip: By default, the CSRF token is sent to the client in a cookie named " `mappend` (decodeUtf8 defaultCsrfCookieName) `mappend` ".\nTip: The server is looking for the token in the following locations:\n" `mappend` T.intercalate "\n" (map csrfLocation expectedLocations)
+csrfErrorMessage expectedLocations = T.intercalate "\n"
+  [ "A valid CSRF token wasn't present. Because the request could have been forged, it's been rejected altogether."
+  , "If you're a developer of this site, these tips will help you debug the issue:"
+  , "- Read the Yesod.Core.Handler docs of the yesod-core package for details on CSRF protection."
+  , "- Check that your HTTP client is persisting cookies between requests, like a browser does."
+  , "- By default, the CSRF token is sent to the client in a cookie named " `mappend` (decodeUtf8 defaultCsrfCookieName) `mappend` "."
+  , "- The server is looking for the token in the following locations:\n" `mappend` T.intercalate "\n" (map csrfLocation expectedLocations)
+  ]
 
   where csrfLocation expected = case expected of
-          CSRFHeader k v -> T.concat ["- An HTTP header named ", k, " ", (formatValue v)]
-          CSRFParam k v -> T.concat ["- A POST parameter named ", k, " ", (formatValue v)]
+          CSRFHeader k v -> T.intercalate " " ["  - An HTTP header named", k, (formatValue v)]
+          CSRFParam k v -> T.intercalate " " ["  - A POST parameter named", k, (formatValue v)]
 
         formatValue :: Maybe Text -> Text
         formatValue maybeText = case maybeText of