Panic when bootstrap downloads fail

Also hints that 32bit snapshots are unavailable on failure

Resolves #3739

fixup! Panic when bootstrap downloads fail
auto merge of #3894 : awestroke/servo/feauture-warn-32bit-bootstrap, r=jdm

Also hints that 32bit snapshots are unavailable on failure

Resolves #3739 

![screenshot from 2014-11-05 13 42 39](https://cloud.githubusercontent.com/assets/1098408/4917781/6edc0dd2-64e9-11e4-98bd-dbd1829ee913.png)