fix(client): use proper exception instead of assert for US endpoints (#1641)

assert statements get disabled when running python with -O flag which
means the region check silently dissapears in production. Replaced with
BinanceRegionException that gives callers a proper catchable error type.

Affects get_staking_asset_us, stake_asset_us, unstake_asset_us,
get_staking_balance_us, get_staking_history_us, get_staking_rewards_history_us