prevent startup if PoA and snap/checkpoint

[macfarla]

Fixes #6385

Docs - can't see anywhere in the docs where we mention snap or checkpoint on the private networks side, but may be worth adding an explicit note that you must use FAST or FULL if using BFT consensus.

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PR description

Fixed Issue(s)

[matthew1001]

Thanks @macfarla, looks good to me, will help avoid confusion.