Should URLs with malformed skylinks return a 401 on Account-Only portals?

[dghelm]

Describe the bug

Non-existent subdomains are interpreted as incorrectly formed skylinks, and on account-only portals, an error is returned about not being authenticated instead of the expected "your skylink is bunk".

To Reproduce

Visit https://asdfasdfsadfsadf.siasky.net/ and see "normal" behavior.
Visit https://asdfasdfsadfsadf.skynetfree.net/ and get asked to authenticate, even when logged in.

Expected behavior

Visiting the page should result in the user being informed their skylink is malformed. For not logged in users, either error will work.
Or, we build a better error page? Or not assume the user is trying to access a skylink when the data doesn't match one of our established server subdomains?

Screenshots