Errors not logged when service fails to change subscription plan

If a user views her subscription tab, hits the Change plan link, clicks on the Select link for a different plan that is currently selected, she gets taken to a Change subscription form with a Change plan button.

On pressing the button, if she gets the following error:

The plan could not be updated because the billing service encountered an error.

...there is no mention of it in the logs, and not explanation anywhere for what the error was.

There should be an error message sent to the log. Patch forthcoming.

The error I'm actually getting is The provided XML was invalid, but I'll worry about that elsewhere, not in this ticket. It could be a problem with the PHP library.