Misleading error message in flattener /catching the wrong exception

[dmoisset]

At

https://github.com/machinalis/featureforge/blob/develop/featureforge/flattener.py#L197 , TypeErrors are caught. The call to next() triggers feature evaluation, and when the call to a feature raises a TypeError, that exception is caught and a ValueError("Cannot fit with an empty dataset") message is generated.

There's no reson to relaunch that exception with that message, the original exception should go through

From what I see in the history, the TypeError guard was there to catch possible problems when calling iter(...), but the call to iter() was moved outside the protected block.