log more at important places in the code

[rudibatt]

1. Define the single values and when they are used:
   • error: only if an error can not be bypassed by a fallback behaviour
   • warn: if there is an error, but also a fallback behaviour
   • info: relevant information about the current process
   • debug: details about relevant information (e.g. parameters)
   • trace: detailed information about everything
2. improve the existing logging by this definition