Attempting to get a market that does not exist results in a strange error

If the market does not exist, you get a KeyError at a strange spot

```
def parse_market(self, data):
    book_price_decimals=data["book_price_decimals"]
...
KeyError: 'asset_id'
```

Before trying to parse, we should check the response data and raise a custom `MarketNotFoundError` or something alike
