improve error messages for empty easyconfigs

[Flamefire]

Currently we get:

ERROR: Failed to process easyconfig foo.eb: No software-specific easyblock 'EB_UNKNOWN' found for UNKNOWN

That isn't exactly clear. So improve in 2 steps:

1. Remove the UNKNOWN constant and just handle in the way we would if the class isn't found: Return None or error if error_on_missing_easyblock is set
2. Check for a fully empty file and error with a clear message

Possible improvement: Pass error_on_missing_easyblock in the EasyConfig constructor and use an empty dict as extra_options if no easyblock_class was found, i.e. self.extra_options = easyblock_class.extra_options() if easyblock_class else {}
This will error later in the parser which doesn't find the required keys like "name".
Not sure if that error ("missing required keys: name, ...") is cleaner than "No easyblock found as neither name nor easyblock were specified" though.

[boegel]

lgtm

[boegel]

I think "No easyblock found as neither name nor easyblock were specified" is a pretty clear error, it's more specific to me than letting it fail later because mandatory easyconfig parameters are not specified.

Thanks a lot for this enhancement @Flamefire!