Support new error-index page

[Folyd]

See also rust-lang/rust#100922

[notriddle]

1. I definitely don't think you want to merge this until the new error index is stable.
2. Even after the new error index is stable, there's a redirect in place so that the old URLs still work.

[Folyd]

1. I definitely don't think you want to merge this until the new error index is stable.
2. Even after the new error index is stable, there's a redirect in place so that the old URLs still work.

Good to know, thanks @notriddle. 😸

[notriddle]

The new error index is stable. If you want to skip the redirect, this can merge now.

[Folyd]

Merged. Thanks for reminding me. 😄 @notriddle