fix the loss of traceback

[ocavue]

this fix removes repr(e) who will lose the stack trace of the original exception

[ocavue]

I removed repr(e) because that will lose my stack trace information. It's hard to debug when I have thirty data loader and the exception only provide 'NoneType' object is not iterable.
I also removed Data loader batch_load_fn function raised an Exception, which is a sweet but unnecessary message. It's easy to know that this error comes from a batch_load_fn function when you know which line raise it.

[syrusakbary]

Thanks for the fix! Merging

[ocavue]

You're welcome