Improve the error message when a user tries to merge accounts on separate sites

Question

If users try to contact us in order to merge two accounts, they will be blocked if they use accounts from two different sites:

Now, stricting speaking, this is a valid profile. It just isn't a profile that we can merge with the other one. We do detect and display less generic errors with other problems. For instance, when I try to use the same profile link I get this error:

the two profile links must be different; please try again

Ideally, if the two links are the same user, but on different sites, the contact form should use this more specific warning. Conversely, if the two links are on different sites, but are not associated, the form should let the message go through since it is possible to merge in that case. (Well, assuming the user is able to show they own both accounts.) Finally, if the links are gibberish or don't have the basic pattern of a profile link, they could be rejected with the current error message.

Answer 1

I've added a few checks to the page and a few more error messages.

For merge requests, it is now possible to post URLs to accounts from other sites.

If the URLs are not valid user profile URLs, an error is shown.

If the URLs are for external (not Stack Exchange) sites, an error to that effect is shown.

If the URLs point to the same (already associated) accounts, an error to that effect is shown.