Add ability to catch libsml error messages

[andig]

...instead of writing them to STDERR

[hmueller01]

You are right. A library should not write to strerr. It should return an error number and the application should decide what to do (write an error message, stop execution, nothing ...) BTW: If you like to suppress the error messages right now you can use sml_server /dev/ttyUSB0 2>/dev/null

[r00t-]

realize that before https://github.com/volkszaehler/libsml/pull/12 it would even write them to stdout.

[r00t-]

made some implementation, please have a look at #93