calibrate: make astrometry failures non-fatal

In many cases, we can limp along without astrometry succeeding, so
there's no need to give up completely.  In some cases, we want to
ensure that we succeed, so added config variable 'requireAstrometry'
to allow a RumtimeError in astrometry to be fatal.

python/lsst/pipe/tasks/calibrate.py

@@ -78,6 +78,11 @@ class CalibrateConfig(pexConfig.Config):
         doc = "Compute photometric zeropoint?",
         default = True,
     )
+    requireAstrometry = pexConfig.Field(
+        dtype = bool,
+        doc = "Require astrometry to succeed, if activated?",
+        default = False,
+        )
     background = pexConfig.ConfigField(
         dtype = measAlg.estimateBackground.ConfigClass,
         doc = "Background estimation configuration"
@@ -201,13 +206,17 @@ def run(self, exposure, defects=None, idFactory=None):
         if self.config.doPsf:
             self.initialMeasurement.measure(exposure, sources)
 
+            matches = None
             if self.config.doAstrometry:
-                astromRet = self.astrometry.run(exposure, sources)
-                matches = astromRet.matches
-            else:
                 # If doAstrometry is False, we force the Star Selector to either make them itself
                 # or hope it doesn't need them.
-                matches = None
+                try:
+                    astromRet = self.astrometry.run(exposure, sources)
+                    matches = astromRet.matches
+                except RuntimeError as e:
+                    if self.config.requireAstrometry:
+                        raise
+                    self.log.warn("Unable to perform astrometry (%s): attempting to proceed", e)
             psfRet = self.measurePsf.run(exposure, sources, matches=matches)
             cellSet = psfRet.cellSet
             psf = psfRet.psf
@@ -240,12 +249,16 @@ def run(self, exposure, defects=None, idFactory=None):
         if self.config.doAstrometry or self.config.doPhotoCal:
             self.measurement.run(exposure, sources)
 
+        matches, matchMeta = None, None
         if self.config.doAstrometry:
-            astromRet = self.astrometry.run(exposure, sources)
-            matches = astromRet.matches
-            matchMeta = astromRet.matchMeta
-        else:
-            matches, matchMeta = None, None
+            try:
+                astromRet = self.astrometry.run(exposure, sources)
+                matches = astromRet.matches
+                matchMeta = astromRet.matchMeta
+            except RuntimeError as e:
+                if self.config.requireAstrometry:
+                    raise
+                self.log.warn("Unable to perform astrometry (%s): attempting to proceed", e)
 
         if self.config.doPhotoCal:
             assert(matches is not None)