Add a check to make sure the sampling time exists before trying to update

For runs cancelled during the marginalized parameter reconstruction,
this can lead to cases where the start_time isn't defined causing an
attribute error. This ensures that doesn't occur.

bilby/core/sampler/dynesty.py

@@ -436,8 +436,9 @@ class Dynesty(NestedSampler):
         logger.info("Writing checkpoint file {}".format(self.resume_file))
 
         end_time = datetime.datetime.now()
-        self.sampling_time += end_time - self.start_time
-        self.start_time = end_time
+        if hasattr(self, 'start_time'):
+            self.sampling_time += end_time - self.start_time
+            self.start_time = end_time
 
         current_state = dict(
             unit_cube_samples=self.sampler.saved_u,