Add revert to enforce timing in dutchAuctionPrice

[jaglinux]

1. Call dutchAuctionPrice() to get the price in dutchAuctionMint()
2. Add revert logic for "if dutch auction has not started yet" in dutchAuctionPrice()
3. Add named return param in dutchAuctionPrice()

Signed-off-by: Jagadish Krishnamoorthy [email protected]