Customer portal: shows 'code sent' for any email, even non-buyers

[stilla]

Bug

from https://app.plain.com/workspace/w_01JE9TRRX9KT61D8P2CH77XDQM/thread/th_01KM3P5M9HWP2WWRE7D5D5Y3E8/

The customer portal shows a 'code sent' confirmation for any email entered — regardless of whether that email belongs to a buyer for that organization.

Expected behavior

If the entered email is not associated with a purchase for that organization, show a clear error message instead of confirming that a code was sent.