Distinquish between error and normal output,

[nhjschulz]

Currently all messages are printed to stdout, regardless if there are errors or normal informal text ouput.

It is common practice since the dawn of current operating systems to seperate errors to the "stderr" stream.
The pyTRLCConverter tool should do the same.

Note: there is a log_verbose module that could be reworked to a generic output handler covering also
normal and error output in addition to verbose message filtering.