Fix anchor.bn truncation, improve invalid asset behaviour and improve market refresh robustness

[filipzeta]

A bundle of a few small fixes:

• 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