_bind_value + Django problem: either use __str__ for the object or__repr__ in the error message
*Created originally on Bitbucket by [d33tah (d33 tah)](https://bitbucket.org/%7Bc309f265-cf26-46be-af23-23925f896779%7D/)*
Hi! Thanks for making this library! :)
I recently had a bug that boiled down to openpyxl saying "Cannot convert <random string> 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
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