[FIX] pos_mercury: selecting vantiv payment during pending payment

When there is a pending payment from a terminal payment method, and we
attempt to add a vantiv payment, an error message is displayed but is
not actually raised in the code. As a result, the code continues with
the effect of highlighting the currently selected payment to be like a
pending vantiv payment. This commit fixes the described issue.

task-id: 1984690