Rework the "no payment gateways are defined" checkout error

Looks good to me.

I've tested the patch and it looks good for me too.

#2917372: incorrect use of translation in CartEventSubscriber tells us that this is the wrong way to do a link inside t(). Let's follow its suggestion.

Good feedback in #5. This looks good to go now.

Core uses : instead of @ for urls.

May be test is failed because you should do this instead of the latest changes:

return $this->t('No payment gateways are defined, <a href=":url">create</a> one first.', [':url' => $add_payment_gateway_link]);return $this->t('No payment gateways are defined, <a href=:url>create</a> one first.', [':url' => $add_payment_gateway_link]);

We need test coverage in a Functional test.

My local is screwing up, here's a pass at a test.

Whoops, forgot the admin permission.

bojanz: thoughts? I wasn't sure at first since it links to admin. But I highly doubt someone will launch like this :) And it improves the developer/site builder experience.

Discussed points:
1) We might have gateways, but none of their conditions passed. "available" is the word we want to use, not "defined".
2) We need to account for the user possibly not having permissions to manage gateways (hence, we need two different messages).

Tweaked as discussed and committed. Thanks, everyone.