Improve return to merchant page

[SiimKirjanen]

Mentioned on: https://wordpress.org/support/topic/stripe-issue-7/#post-17365891

Current return to merchant page looks ugly and can cause confusion with clients.
"Anything would be better than that ugly white page that the first thing it suggests is that there’s been a error during the payment or something, which is not a great impression for the booker…"

Think how to improve the page and implement it.