Catch invocation errors during openvpn launch

We need to be able to notify a bad openvpn invocation. This might be due to a malformed openvpn command, missing files, etc. or also by a server timeout.

For all the cases, maybe catching a return != 0 is the easiest to begin with.

We have several main paths here:

• Implement a callback for the VPNProcess.processEnded (easiest?) and map to a qt signal.
• Implement again an observer for the output of the openvpn process. If we end up daemonizing the client, we could attach to the management interface and ask for the nth last log lines.
• Implement a timeout (maybe when we get the MissingSocketError) and raise after a maximim time/tries.

(from redmine: created on 2013-06-16, closed on 2013-06-23, relates #948 (closed), duplicates #1270 (closed), precedes #2956 (closed))