Redirect to latest version does not work if build failed

[jyn514]

This is actually a combination of a number of issues:

1. If you go to https://docs.rs/isatty/0.1.0/isatty/, the link to the latest version takes you to https://docs.rs/isatty/0.2.0/isatty/?search=isatty
2. isatty 0.2.0 failed its build
3. /isatty/0.2.0/isatty/ gives a 404

#325 fixed this for /isatty/0.2.0/isatty, so the issue is that /:crate/:version/:target/ behaves differently than /:crate/:version/:target.

We can't just change the redirect to link to /:crate/:version/:target, since ?search= doesn't work on that page. Maybe the easiest thing would be to link to /:target and have the redirect to /:target/ keep the search query parameter?

[jyn514]

Perhaps it would be a better idea to not show 'go to latest version' for failed builds? That would hit the majority of use cases. For this particular case though I think we really should show 0.2.0 so people see the 'this crate is deprecated' message.