fix: make it impossible to overwrite default key

Replacing default key
when a profile is already part of
verified groups results in
`[The message was sent with non-verified encryption. See 'Info' for more details]`
messages for other users.

It is still possible
to import the default key before
Delta Chat generates the key.

python/src/deltachat/message.py

@@ -215,7 +215,7 @@ class Message:
         """extract key and use it as primary key for this account."""
         res = lib.dc_continue_key_transfer(self.account._dc_context, self.id, as_dc_charpointer(setup_code))
         if res == 0:
-            raise ValueError("could not decrypt")
+            raise ValueError("Importing the key from Autocrypt Setup Message failed")
 
     @props.with_doc
     def time_sent(self):