Payment Request - Allow payment handlers to report back internal errors

### WebKittens @marcoscaceres ### Title of the proposal Payment Request - Allow payment handlers to report back internal errors ### URL to the spec https://github.com/w3c/payment-request/pull/1050 ### URL to the spec's repository https://github.com/w3c/payment-request/ ### Issue Tracker URL _No response_ ### Explainer URL _No response_ ### TAG Design Review URL _No response_ ### Mozilla standards-positions issue URL _No response_ ### WebKit Bugzilla URL _No response_ ### Radar URL _No response_ ### Description Salut, WebKit friends :). I'm writing to request an official standards position from WebKit on a minor change to Payment Request, to allow payment handlers to report back internal errors. Marcos has already been involved in the design + review, but [I was still asked to file this](https://groups.google.com/a/chromium.org/g/blink-dev/c/j8D3S2ZyozQ/m/r3Um43y1BwAJ) in our launch process, so here I am :).