So users are pointed to some random page (either Phabricator or on-wiki) to "report the issue" (without any hints how to do that in an effective way), and without any automatic passing of e.g. the problematic image URL or without showing or even passing any actual error message?
I don't think that helps anybody. Neither reporters nor editors.
Wherever a feedback venue is it should be where someone (developers) actually takes a look (and with (automated?) instructions how to provide sufficient information for developers). But if noone takes a look it does not matter if it's Phabricator or a wiki page (see the depressive UW feedback page example on Commons) and I'd rather disable such feedback links entirely instead of giving users a fake feeling that someone looks at their feedback.
The current error page looks like this, and the reporting URL just takes you to a new task form in Phabricator with only the project field filled out.
One option is to just get rid of it. Another one is to add error details; the error message can be added easily, but for reasons outside MediaViewer, it is rarely helpful (e.g. connection errors tend to just say 'http'). URL and browser UA are also easily added but could be a privacy issue. We would also want to show some explanation on how to write meaningful error reports. Also, the majority of users don't have a Phavbricator account and are redirected to the OAuth registration process; we would have to verify that pre-filled form fields do not get lost when that happens.