The code had set flag_edge if the source was off the chip, but the
docstring did not specify that.

There is no Box2I.contains(Point2D), so we have to cast the box instead
of the centroid, if we want it to work for non-finite values.

Catching NaN centroids early means that the offimage flag is not set
for them, but it should be.
Reformat exception message to be more useful (although the measurement
task squashes all these to debug level logs, so noone ever sees them
anyway).
Add tests of non-finite centroids and sources right on the image boundary.