Log exception in case of an error during tag calculation.

ccnode/tags.py

@@ -58,6 +58,7 @@ class Tag(object):
         try:
             self.value = self._calculate_value()
         except Exception:
+            logger.exception('Cannot calculate tag value for %s', self.name)
             self.value = None
         # logger.debug('Calculate Tag(%s) = %s', self.name, self.value)