Factor out parameter error checking

[Samir Yitzhak Gadre]

Currently parameter error checking and throwing is done on-the-fly in main.py. This means we may do some heavy weight initialization (e.g., of model) only to throw if a user passed incompatible flags. Having an error checking function, called after argparse, will alleviate this and also clean up code.

[Igor Vasiljevic]

@achalddave looking into this

[Achal Dave]

#153 closes this