JDBC and JNDI data sources have a Test function. This allows the user to save the definition even if it won't work right now. While not elegant, it is reasonable to do this, so currently entered definitions can be saved, retrieved later and corrected.
Custom data sources definitions have a validation function, which can be used to test the data source. If the validation fails, you cannot save the information.
The request is to have a Test function available that does the validation and shows errors and a separate Save function.