Return Nan or set another flag for negative second moment

Comment on Robert on Slack (auto-guiding channel)

The object
pfs_visit_id = 126001
agc_exposure_id = 707834
agc_data_flags = 0x29
agc_camera_id = 2
(centroid_x_pix, centroid_y_pix)  = (604.57526, 165.96841) 
mxx, mxy, myy = 1.0728481, 5.941382, -4.3275137 

has a negative second moment, <y^2>.   The 0x8 bit is set indicating that the adaptive moments failed, and probably the bright trail confused your object detection, but second moments should never be less than zero.  I think that this is probably 'central_image_moment_02_pix, which is set from x2 as returned by sep.extract.

Please fix this.  If you don't want to fix sep,  please set an new flag indicating a SExtractor error, and/or set the values to NaN.

At the same time, please provide an API for users to get an enumerated type identifying all the error flags which you set.  I think that the current list is

agccFlags = {0: 'right',
             1: 'edge',
             2: 'satur',
             3: 'bad_shape',  # from windowedFWHM()
             4: 'ellip',
             5: 'flat_top',
            }