Change the 'Model not available' behavior

Currently when a model is not available, it will delete my massage and paste it back into the input box
But that hardly works as expected. It will delete my message during regenerations, it will ignore my edits and delete them when the model is not available, etc

Please just copy everyone else and return an error message where the model message would go. The current implementation is janky and has lost me many prompts I had to retype from memory