_bind_value + Django problem: either use __str__ for the object or__repr__ in the error message

Created originally on Bitbucket by d33tah (d33 tah)

Hi! Thanks for making this library! :)

I recently had a bug that boiled down to openpyxl saying "Cannot convert to Excel". It turned out that my project was passing a Django object to openpyxl and the error message contained its __str__. Could you either use __repr__ in the error message so it's easier to debug or - preferably - try calling __str__ on an unknown object instead of crashing?

Cheers, d33tah