BUG: Bad error message when constructing memoryview from structured view

[eric-wieser]

As instructed by the error message:

data = np.zeros(1, dtype=[('a', int), ('b', float)])
data = data[['b','a']]
memoryview(data)
# RuntimeError: This should never happen: Invalid offset in buffer format string generation. Please report a bug to the Numpy developers.

I don't think this can be fixed, but the error message can certainly explain that a field reorder is required.

cc @ahaldane due to #6053