Provide a useful error message if the EXERCISM_CONFIG_FILE points to a directory

[kytrinyx]

Someone reported the following:

I get a message saying 'The handle is invalid' when doing anything

It turns out they had put a folder as the EXERCISM_CONFIG_FILE instead of the actual file

[lcowell]

I think I can take this on. I'll be using the same mechanism to turn XDG_CONFIG_HOME in to a full config path.

[kytrinyx]

nice, thanks