Improve the warning on nddata.NDUncertainty.propagate

[hayesla]

Description

Currently if a user tries to propagate errors (i.e. nddata.NDUncertainty.propagate) with an operation not currently supported (e.g. np.power) it will error with a ValueError

>>> ValueError: unsupported operation: power

however, it would be better to be a NotImplementedError?

[neutrinoceros]

Thank you for reporting !
I agree that this error is a bit confusing and that NotImplementedError would make more sense here
For convenience, here's the context it's emitted from:

astropy/astropy/nddata/nduncertainty.py

Line 404 in 0d4fc36

raise ValueError(f"unsupported operation: {operation.__name__}")