Better get_input_entity code flow

Plenty of unnecessary exceptions were being raised and caught when
the input parameters were already correct. Furthermore, since
8abc7ade22, the in-disk cache was no
longer being used (so using usernames always reached out to memory).

telethon/entitycache.py

@@ -12,11 +12,12 @@ class EntityCache:
         Adds the given entities to the cache, if they weren't saved before.
         """
         if not utils.is_list_like(entities):
-            # Invariant: all "chats" and "users" are always iterables
+            # Invariant: all "chats" and "users" are always iterables,
+            # and "user" never is (so we wrap it inside a list).
             entities = itertools.chain(
-                [getattr(entities, 'user', None)],
                 getattr(entities, 'chats', []),
-                getattr(entities, 'users', [])
+                getattr(entities, 'users', []),
+                (hasattr(entities, 'user') and [entities.user]) or []
             )
 
         for entity in entities: