config: remove accidental scheme validation circumvention, improve errors, warn on config loading failure in main()

This will fail on values of the wrong type, but still warn about and allow unknown keys.

main.py

@@ -30,6 +30,8 @@ def cli(verbose: bool = False, config_file: Optional[str] = None, wrapper_overri
     config.runtime.no_wrap = wrapper_override is False
     config.runtime.error_shell = error_shell
     config.try_load_file(config_file)
+    if config.file_state.exception:
+        logging.warning(f"Config file couldn't be loaded: {config.file_state.exception}")
     if wrapper_override:
         enforce_wrap()