I see one obvious issue with the patch: The ConversionErrors it creates are passed the struct.error or TypeError instance as a parameter. The first argument of these exceptions would be better, i.e.

try:
   ...
except struct.error as e:
   raise ConversionError(e.args[0])

Furthermore, my ear thinks that raises_conversion_error would be a better name for the decorator than raising_conversion_error.

Anyway, I think the whole ConversionError is a bit problematic, as either TypeError or ValueError would be the most appropriate exception, depending on the situation. For example:

p = Packer()
p.pack_int('foo')  # should raise a TypeError
p.pack_int(2**100) # should raise a ValueError

This would be slightly harder to implement, though, as struct.error has exactly the same problem.