Fix anchor.BN truncation in risk calcs: small accounts will truncate uPnL to 0, causing accuracy issues with trade size and liq price calcs
Replace throw with an undefined return for invalid asset: throwing is a bit too aggressive, and caused some issues in FE
Add check for Exchange.isInitialized when refreshing markets: if loading is super slow then the .isHalted() check can cause syntax errors because Exchange.state is undefined
A bundle of a few small fixes: