Avoid pulling core::fmt for PackingError when unwrapping

[jedrzejboczar]

In my comment I said that all the fmt was being pulled due to my application code, but I actually didn't notice that I was testing with slight modifications to usbd-human-interface-device. I reviewed my changes and it looks like fmt was being pulled due to .pack().expect(...) (besides my unwrapping of UsbHidError).

This PR fixes the problem by dropping the PackingError before calling expect. I also included a minor fix that avoids hex formatting in one panic!. I think that the fixed-string message should give enough context (you could add log::error! if more context is needed).