Bug 1087674 - Handle XHR abort()/timeout and certificate errors more gracefully in GMPInstallmanager. r=gfritzsche, a=sledru

Prior to this patch, a Man in the Middle (MITM) attack on SSL could cause GMPInstallManager to fail during the check for updates, which in turn would cause a crash during shutdown. This was observed in the wild by users of recent versions of Avast, which performs such attacks on SSL as part of its "HTTPS scanning" feature. With this patch, errors are handled more gracefully. The attack still prevents any update (including the install of OpenH264) but at least it does not cause a crash anymore.