Returning error instead of raising when a transaction is not mined.

[Miguel Diaz]

At the moment when you try to check the receipt of a transaction tx_receipt/1 with a hash of a transaction which has not been mined yet, the function will raise a BadMapError I think it should be better to return an error ({:error, :not_mined} in this case) so we don't cut operations of related functions outside of the library,