Revert "Better error message in case a trackable module can't be saved."

This reverts commit 43d0715238.

save() returns false only when validations failed. In this case, validations are
not performed. Therefore save() may never return a falsy value.
If save() fails, the appropriate exception is raised.

With certain ORMs, such as NoBrainer, save() never returns true/false, but
always raise an exception. This commit lift the incompatiblity.

lib/devise/models/trackable.rb

@@ -30,8 +30,7 @@ module Devise
 
       def update_tracked_fields!(request)
         update_tracked_fields(request)
-        save(validate: false) or raise "Devise trackable could not save #{inspect}." \
+        save(validate: false)
-          "Please make sure a model using trackable can be saved at sign in."
       end
     end
   end