Validate GENESIS_VALIDATORS_ROOT earlier

[remyroy]

This is part of the last comments from the recent ToB audit in improving usability.

A couple of small sad-path performance/usability improvements to consider:

• Validate GENESIS_VALIDATORS_ROOT earlier so that, if it's eg None, we fail before getting into user interactions

[valefar-on-discord]

Would this not break Ephemery usage as its chain settings has GENESIS_VALIDATORS_ROOT hardcoded to None