When ooniprobe starts it should perform checks to verify the config file is correct

This issue was automatically migrated from github issue https://github.com/TheTorProject/ooni-probe/issues/279.

There are some cases that can be detected as wrong settings that should fail gracefully at startup.

For example if you have set start_tor to false and no socks port is set ooniprobe should fail to start and output a comprehensible error message.

Also when the socks_port is set it should verify that Tor is listening on such port.