Change snap-sync UX for early history.

[shamil-gadelshin]

This PR adds a hard error for the attempt of snap-sync during the early history and makes the full snap sync mode default. Previously, snap sync attempts continued with errors: we don't have genesis state on snap sync and new blocks were unexpected because snap sync was skipped along with new state downloading.

Code contributor checklist:

• I have read, understood and followed contributing guide

[nazar-pc]

I don't like this and don't think it is necessary. We need to fix it properly in Substrate and use --sync full in the meantime explicitly.

[teor2345]

I think it would help users to set the default to full, so it works from genesis with the current code. But I agree that fixing it properly is the way to go.