Add proper failure messages for common failures

[Melkor333]

If the configuration file is "wrong", it leads to seemingly fatal crashes, like the following when the configuration doesn't have any snapshots defined:

# pyaptly publish config.toml create
ERROR: snapshot
               Wrote traceback to: /tmp/tmpwpp159mr

For better UX, it should explain what is missing/wrong... I assume that's something which exists in various places.