improve returned data if running a simulation (using the model executable)

[syntron]

see #108

[OMCSession] improve log messages for model simulation using OM executable

• use check=False => no CalledProcessError exception; possibility to handle the return code / error code
• error code needs to be checked (compare == 0) by the caller!
• improve log messages; print returncode if it is != 0 with stdout

[ModelicaSystem] improve handling of model simulation

• ensure a message if logged if returncode != 0