Steps to replicate the issue (include links if applicable):

• Create a broken redirect, e.g. from [[Foo]] to [[Foobar1234567890]].
• Note these can occur in poorly-maintained wikis.
• Try to create a redirect from [[Bar]] to [[Foo]].

What happens?:

• T326056: Show a warning on creating double redirect is correctly thrown, but the error message recommends redirecting the page to [[Foobar1234567890]], which doesn't exist.
• If this advice is followed, then it next throws an error per T326057: No warning on creating broken redirect to red link.

What should have happened instead?:

DoubleRedirectConstraint should check the target it wants to suggest actually exists before suggesting it. If it doesn't exist, then it should throw an error message like:

⚠️You are trying to redirect a page to a target which is a broken redirect. Please reconsider your actions.