fixes bug 1393472 - KeyError on 'caused_by' in RequestError handling (#3940)
fixes bug 1393472 - KeyError on 'caused_by' in RequestError handling (#3940)

* fixes bug 1393472 - KeyError on 'caused_by' in RequestError handling

* raise the right exception context

* excess sys import

* comment typo

Tayamarn/socorro,m8ttyB/socorro,Tayamarn/socorro,lonnen/socorro,Tayamarn/socorro,lonnen/socorro,Tayamarn/socorro,m8ttyB/socorro,mozilla/socorro,m8ttyB/socorro,mozilla/socorro,Tayamarn/socorro,Tayamarn/socorro,mozilla/socorro,lonnen/socorro,mozilla/socorro,m8ttyB/socorro,m8ttyB/socorro,mozilla/socorro,mozilla/socorro,lonnen/socorro,m8ttyB/socorro