Fix internal server error regressions

[elboulangero]

If the database is not reachable for some reason, mirrorbits must not return "500 Internal Server Error", instead it must redirect users to the fallback mirror(s).

This was broken in ed1db9e, here's the fix.

The pull request also adds copious unit testing for the http mirror handler.

[jbkempf]

This is serious.

We need to do another release this quarter if we find more regressions…

[elboulangero]

I added a minor improvement at #197. I'd be in favor of doing a new release after that, whenever you want. I can update the CHANGELOG if you want, just ping me. Thanks!

[jbkempf]

I added a minor improvement at #197. I'd be in favor of doing a new release after that, whenever you want. I can update the CHANGELOG if you want, just ping me. Thanks!

I will need that, yes.

[elboulangero]

@jbkempf Do you want to cut a release? I'm waiting for that before I can open new MRs.

Moreover I noticed that Tails switched to 0.6 already: https://download.tails.net/tails/?mirrorstats

[jbkempf]

@jbkempf Do you want to cut a release? I'm waiting for that before I can open new MRs.

I think we should. But you shouldn ´t wait to open new MRs, tbh :D

[jbkempf]

Done.