Provide better feedback when output_dir not found

[toliwaga]

If output_dir is not found, output_dir method in core/config.py

raise RuntimeError("output_dir: directory does not exist")
resulting in the message:
RuntimeError: output_dir: directory does not exist

This is misleading because 'output_dir' is the name of the method raising the error, not the expected name of the directory it isn't finding. Instead, or in addition, should print path that wasn't found. e.g. something like:
RuntimeError: output_dir: directory does not exist: F:\some_path\output

[bstabler]

#397