core: better handling of profile_key and don't write the param file anymore

- new error ProfileNotSetError and some methods use the default @NONE@ instead of @DEFAULT@
- do not output the .sat/param file anymore, it is not needed and created confusion
- plugin XEP-0054: remove an error message at startup

src/core/exceptions.py

@@ -26,6 +26,13 @@ class ProfileNotInCacheError(Exception):
     pass
 
 
+class ProfileNotSetError(Exception):
+    """
+    This error raises when no profile has been set (value @NONE@ is found, but it should have been replaced)
+    """
+    pass
+
+
 class NotConnectedProfileError(Exception):
     pass