fix crash when editing name of model

- fixes a spurious "unknown model name" error when trying to edit the
  short name of an existing model.
- relaxes naming requirements to include the ':' and '/' characters
  in model names

ldm/invoke/model_manager.py

@@ -125,7 +125,7 @@ class ModelManager(object):
         Set the default model. The change will not take
         effect until you call model_manager.commit()
         '''
-        assert model_name in self.models,f"unknown model '{model_name}'"
+        assert model_name in self.model_names(), f"unknown model '{model_name}'"
 
         config = self.config
         for model in config: