refactor: handle error in fetch_order_book() in the same way

[psyberck]

What this PR adds (Close Issue #66 )

When an OrderNotFound exception is caught, it now returns an empty OrderBook(bids=[], asks=[]) instead of re-raising the error — making it consistent with how other fetch methods handle "not found" cases.

Other exceptions will still be re-raised

Testing result

exchange = pmxt.Polymarket()
p = exchange.fetch_order_book('non-existing-orderbook')
print(p)

OrderBook(bids=[], asks=[], timestamp=None)