Slicing out of logical bounds returns unclear error

[BrianMichell]

If a user slices outside of the logical bounds the current error is not clear what the issue was.

It should be enhanced to display the bounds.

https://github.com/TGSAI/mdio-cpp/blob/838538df4bd796994e656126767b379d2ca7d904/mdio/variable.h#L1095C7-L1100C44