fix 109, rework exception handling

[redshiftzero]

Fixes #109

Right now we have these status codes provided by the securedrop-proxy qrexec service: https://github.com/freedomofpress/securedrop-proxy/blob/21c2963ffb383092142bafdda6fd5886857b675d/securedrop_proxy/proxy.py#L263 and https://github.com/freedomofpress/securedrop-proxy/blob/21c2963ffb383092142bafdda6fd5886857b675d/securedrop_proxy/proxy.py#L270 which we should be passing through for clients to use.

(btw, we will probably treat a ServerConnectionError the same as a RequestTimeoutError from securedrop-client, but we shouldn't make them the same from the SDK since we might want to customize behavior later)

To test:

1. Run client on branch use-sdk-serverconnectionerror
2. Install this branch of securedrop-sdk instead of 0.0.12
3. Open client, connect, wait for sync. It should succeed.
4. Pause staging server.
5. You should NOT see the logout window come up as described here.
6. Send a reply.
7. See "The SecureDrop server cannot be reached" with a retry button.

Once this is merged we can make an sdk release

[sssoleileraaa]

I ran through the test plan and it fixes the issue