Fallback to old download methods if server returns 403

[brson]

Fixes #452

Can't be tested because no network tests.

r? @alexcrichton

[est31]

Nice to see a fix being proposed so fast :)

[jethrogb]

Why don't you just test for all 400 codes?

[brson]

@jethrogb That sounds good to me if you want to submit a patch.

[est31]

Can there be a new release?

[masklinn]

@brson since server changes have already failed rustup.sh several time, wouldn't it be possible and simpler to generate the relevant manifests for older releases (at least the "release" channels anyway) and avoid the fallback?

[brson]

@est31 I'm working on getting a new release out.

@masklinn That approach is possible, yes. It hasn't seemed worth it since this is also simple and the code already exists.