[MAKE.py] additional error handling when missing an option in private_settings.py

Adding an error handling.

make.py with the option -n PROGRAM_NAME wants to have a test_alias
variable in private_settings.py. Without this alias and with a wrong
PROGRAM_NAME an error message is generated because of a missing else
branch.
Merge pull request #568 from ohagendorf/make_py

Tools: make.py - additional error handling when missing an option in private_setttings.py