7198, 7212: attach invalid date to DateError

when Date.set throws during the date fields sanity
check validation, it now attaches an (undocumented) .date field
to the exception -- proof of concept for 7198 and potentially
for 7212 as well

gramps/gen/lib/date.py

@@ -1643,9 +1643,10 @@ class Date(object):
                             raise DateError("Invalid year {} passed in value {}".
                                     format(original, value))
 
-                except DateError:
+                except DateError as e:
                     log.debug("Sanity check failed - self: {}, sanity: {}".format(
                         self.to_struct(), sanity.to_struct()))
+                    e.date = self
                     raise
 
     def recalc_sort_value(self):