BUG: Exception will be raised when nan value sent to order()

Previously order was not checking for nan values sent as
limit or stop prices. It will now raise a runtime exception
in the event that an attempt to order with a nan price is made.
This message might be a little confusing for users.  They did pass in a "number" in some sense...it just happened to be the special number value that means "not actually a number".  I think this might be clearer as:
`Attempted to place an order with a limit_price of NaN`.