No error message when saving a structure containing uppercase or other invalid characters

No error message appears in the chat or game log when attempting to save a structure that contains uppercase characters. The structure file also doesn't appear in the "generated" folder if that's the case.

To reproduce place down a structure block, switch it to SAVE mode, input a name containing an uppercase character, click "Done". Come back to the GUI to press "Save". Notice no error message appearing and no structure file being generated.

This leads to confusion and (probably) bug reports similar to MC-122831