Update server_ctl.py to work with Python 3

Line 16 of server_ctl.py uses the Python 2 syntax for handling exceptions:

except subprocess.CalledProcessError, error:

This should be changed to:

except subprocess.CalledProcessError as error:

Which will work in both Python 2 or 3:

Also in Python 3, the process object cannot be pickled on Linux. The code from Windows that pickles the process ID can be used on Linux as well.