[Enhancement] Proper logging and error handling

[iwilltry42]

Currently we're using the standard logging library and at some points at the code we check for the --verbose flag, while at other points we don't.
Also we don't have any sorts of log level (just print or fatal) between which we could switch.
Additionally, the log messages differ highly from each other.

Bonus: our error handling is quite mixed... at some level we pass the error upstream, sometimes we drop out with a Fatalf or we simply log something and go on. This could be approved e.g. in a way that we only drop out of the process at the top-most level and have lower-level functions only returning meaningful errors.

It would be nice if we could

• clean up the log messages to a unified format
• introduce a more feature-rich logging library (e.g. sirupsen/logrus)
• Get proper unified error handling

[iwilltry42]

Better logging introduced by @nlamirault . Postponing new error handling until next version.