Perform checks before running wraith latest

[ChrisBAshton]

Wraith should check for the existence of the history_dir property in the YAML file, the existence of the corresponding directory, and the existence of all images in that directory (which should match up to the paths provided in the YAML file).

Currently, Wraith will diligently run and take screenshots, only erroring at the end of the process when the history_dir does not validate in some way.