Adds a warning if there aren't enough funds to cover interchain gas payments. Atm this is pretty late in the tx flow and quite ugly. We should probably consider adding a "fees" section to the main tx builder UI bit so this is extra clear -- I anticipate this being a point of confusion. Fees to go to BSC are about 15 ZBC and to Solana about 10 ZBC (ZBC is about 1 cent in value)