Disallow fitBackground returning None

Make SubtractBackground.fitBackground raise RuntimeError
instead of returning None, unlike the getBackground
function it replaces.

examples/subtractBackgroundExample.py

@@ -31,7 +31,7 @@
 import lsst.afw.math as afwMath
 from lsst.meas.algorithms import SubtractBackgroundTask
 
-def loadData(psfSigma=1.5):
+def loadData():
     """Load the data we need to run the example"""
     mypath = lsst.utils.getPackageDir('afwdata')
     imFile = os.path.join(mypath, "CFHT", "D4", "cal-53535-i-797722_small_1.fits")

python/lsst/meas/algorithms/subtractBackground.py

@@ -215,8 +215,6 @@ def run(self, exposure, background=None, stats=True, statsKeys=None):
 
         @return an lsst.pipe.base.Struct containing:
         - background  full background model (initial model with changes), an lsst.afw.math.BackgroundList
-
-        @throw RuntimeError if fitBackground returns a background of None
         """
         displayBackground = lsstDebug.Info(__name__).displayBackground
 
@@ -226,8 +224,6 @@ def run(self, exposure, background=None, stats=True, statsKeys=None):
         maskedImage = exposure.getMaskedImage()
 
         fitBg = self.fitBackground(maskedImage)
-        if not fitBg:
-            raise RuntimeError("Unable to estimate background for exposure")
 
         maskedImage -= fitBg.getImageF()
         background.append(fitBg)
@@ -266,8 +262,10 @@ def fitBackground(self, maskedImage, nx=0, ny=0, algorithm=None):
         @param[in] ny  number of y bands; if 0 compute from height and config.binSize
         @param[in] algorithm  name of interpolation algorithm; if None use self.config.algorithm
 
-        @return fit background as an lsst.afw.math.Background, or None if the estimation fails
-                    (unless the code raises an exception)
+        @return fit background as an lsst.afw.math.Background
+
+        @throw RuntimeError if lsst.afw.math.makeBackground returns None,
+            which is apparently one way it indicates failure
         """
         if not nx:
             nx = maskedImage.getWidth()//self.config.binSize + 1
@@ -344,4 +342,7 @@ def fitBackground(self, maskedImage, nx=0, ny=0, algorithm=None):
                                                self.config.weighting)
             bctrl.setApproximateControl(actrl)
 
-        return afwMath.makeBackground(maskedImage, bctrl)
+        bg = afwMath.makeBackground(maskedImage, bctrl)
+        if bg is None:
+            raise RuntimeError("lsst.afw.math.makeBackground failed to fit a background model")
+        return bg