Error handling

[elmbeech]

most of the Error write to std::cout only and give no error code back.
usually PhysiCell then just quits.

a Errors should be written to std::cerr and a proper Error should be raised (giving -1 back),
so that e.g. job scheduler like slurm can detect that there was an error and the code not successfully ended.

replacing:
std::cout << "Error: message goes here!" << std::endl;
with:
std::cerr << "Error : message goes here!" << std::endl; exit(-1);
should do the trick.

most easy way to find all error messages is: grep -r Error or grep -ir Error

happy to fix this and do a pull request.
please let me know. Elmar