fix: Better user-facing error messaging on server error

When a server error occurs, we want to make sure that the error message displayed to the end user is something helpful for them - which differs from what might be useful to us, as the admins and devs. We need as much technical detail about the error as possible; users need a way to retry the bugged action, or a place to lodge a complaint that the website didn't work, and to move on with their day. This is why both the Paypal and Stripe controllers contain a custom error message to be served back to the front-end.

Previous versions of this custom user-facing error message suggested that the user email "admin@torproject.org" with any show-stopping issues. This is not actually the correct action to take; rather, there is a helpful Tor Wiki page which outlines the various modes of support-related communication which would be relevant in this scenario: https://gitlab.torproject.org/tpo/tpa/team/-/wikis/support

The user-facing error message has been updated to instead point the user to this wiki page. (While support.torproject.org also exists and is quite helpful in many ways, it is not likely to provide any specific insight into a web form breaking due to, like, a third-party's API outage, so we elect instead to point the user towards more relevant places to point out such an issue, such as IRC, the GitLab issue tracker, etc.)