fix: Don't crash on invalid config import

Currently if you pass a bogus JSON file to the `config import` command,
you get a nil deref.

Detect this and fail with a helpful error message instead.

cmd/config_import.go

@@ -53,6 +53,10 @@ The path must be for a json or yaml file.`,
 			return err
 		}
 
+		if file.Settings == nil || file.Server == nil {
+			return errors.New("invalid configuration file: 'settings' or 'server' fields are missing. Please ensure you are importing a file generated by the 'config export' command")
+		}
+
 		file.Settings.Key = key
 		err = st.Settings.Save(file.Settings)
 		if err != nil {