Force retry error message if error hangs logout (bug 886742)

This adds a retry error message if something hangs up the logout. Mainly this would be something like bango having a 500 which doesn't cause a failBack in the request. 

To test it then there's this patch [1] which makes the bangoLogout fail the first time around and succeed the second - just adapt the mp.mozilla.dev line to point at your webpay (I found this works best if you set it to a different host than the way you're requesting webpay - I think it's that that's at the root of causing the non-failure in the 500 when the data type is script as it fails as expected if the host matches the request).

The default timeout is 30s which may or may not be too long - I'm happy to take suggestions on that.

[1]  https://gist.github.com/muffinresearch/1a4675c2d39e3e7c0ee4