Do not add extension to confname when user-defined

If the user chose the confname, we should generate an error rather than
silently change it when we find it's already taken.

commands/add.py

@@ -109,6 +109,14 @@ def run(args, persoconf, logger):
     # Check the conf file name
     if (    confname == persoconf.metafile
          or confname in appmeta.files ):
+
+        # If the confanme was user-defined, it's a serious error.
+        if args.confname:
+            logger.error( "Confname '%s' is already taken: try another one"
+                            % confname )
+            exit(1)
+
+        # Otherwise it's cool, we just have to change the confname by ourselves
         logger.info( "Confname '%s' is already taken: trying another one"
                         % confname )