about:failure doesn't exist any more

[jdm]

This disappeared along with the pre-fetch network stack; now basic_fetch only checks for about:blank. This explains why we sometimes end up seeing "Unexpected scheme" error pages.

[delan]

Adding a case for about:failure to scheme_fetch in components/net/fetch/methods.rs should get us most of the way, though ideally we want to have a reload button and/or keep the location bar pointing to the original url.

Chrome’s behaviour:

[delan]

• we navigate to about:failure without replacing, which trashes the forward history
• even if we navigated with replacement, we would trash the original url and LoadData
• we should instead “reload” the page to a synthesized error page like BadCertHTML