Improve `WrongDescriptor` error message by printing the expected descriptor

[flip1995]

Currently, when reading (or writing) an NPZ file and the type descriptor is not as expected, the error message only shows the descriptor that's in the NPZ file:

ndarray-npy/src/npy/mod.rs

Lines 488 to 490 in 169c116

	ReadNpyError::WrongDescriptor(desc) => {
	write!(f, "incorrect descriptor ({}) for this type", desc)
	}

This could be extended by , expected ({}) showing what descriptor was expected.

I don't know if every place in the code that produces a WrongDescriptor error also knows what the expected descriptor is though. So not sure how feasible this request is.

[jturner314]

There are multiple possible descriptors for each element type, since the endianness can be specified with <, >, or =, and multiple labels can refer to the same type (e.g., u4 and uint32). See https://numpy.org/doc/stable/reference/arrays.dtypes.html for details. I suppose the error message could list out all of the supported descriptors, but I'm not convinced that would be worth it.