Explicit error message, if solution is not accepted

[josojo]

https://github.com/gnosis/dex-contracts/blob/bcf94f72947f76c1e55fceb35f53c6b0e3e73aff/contracts/BatchExchange.sol#L319

This revert message is a little confusing because doesn't tell apart between "this batch is old", "this batch is in the future" and maybe "this batch is the current batch but it's already after the fourth minute"

It would be easier for other developers if the error would be explicit