Separate log messages from stdout

[Vincinator]

What would you like to be added:

• Add more log messages (different log levels)
• Do not print log messages to stdout, stdout should be usable for bash pipes
• set exit codes

Why is this needed:

• this improves usability and presentability of this tool
• required for Integration of OCI delivery in gardenlinux build pipelines #15, so that stdout is clean

[Vincinator]

Mostly done. Need #15 first, so we can check if logging works well with GitHub pipelines (log file path, granularity of stdout, exit codes)