Inconsistent tuple lengths in find_ellipse

When testing in gwcelery, an error was found in find_ellipse where the expected values to unpack were different from expected:

This appears to occur due to the tuples returned being different lengths, with the standard output being six values (https://git.ligo.org/lscsoft/ligo.skymap/-/blob/ba63039d857d5097417e5d967d7d6db185327d5f/ligo/skymap/postprocess/ellipse.py#L387) and the error output being five (https://git.ligo.org/lscsoft/ligo.skymap/-/blob/ba63039d857d5097417e5d967d7d6db185327d5f/ligo/skymap/postprocess/ellipse.py#L371)

This could be fixed by changing the latter output to six np.nan values.