config_load should error (by default) instead if file is not found

[PGijsbers]

Currently when loading/specifying a file in the configuration, e.g., a framework file, if there is a typo the file is ignored and a warning is given. It would be a lot easier to track issues down with e.g., spelling errors (which are not checked in the yaml configuration), if the benchmark would just stop right there with an error.

I am not sure why a warning is given instead of an error. This warrants some investigation, perhaps this is to make some special run mode or some preconfigured defaults work (like user configurations). Should check this before changing the behavior. Alternatively it could stay warning by default and optionally error through a configuration option.