Fix json logger

[thorinaboenke]

This PR fixes the json logging broken in PR #125 Reason: json.dumps only work with basic datatypes

• recursively makes commands serializable for json logging (the loop command contains a list of commands)
• adds informative error message to json logging
• replaces deprecated logger.warn with logger.warning throughout the project

example structure:

The actual execution of the commands (+time) gets logged as well in separate entries