Errors should not be printed to stdout

[tobiiasl]

Use case: Piping output to json will produce confusing error messages in case of e.g. a parse error in the katt script.

Errors should be printed to stderr instead of stdout.

Katt version: c41881449c1a4459f1b410006e1114f42f12b272 (1.5.4-29-gc418814)