Catch signal error in dynesty

This was preventing dynesty from running on Windows systems because these signal attributes don't exist on Windows systems.

bilby/core/sampler/dynesty.py

@@ -123,10 +123,13 @@ class Dynesty(NestedSampler):
 
         self.resume_file = '{}/{}_resume.pickle'.format(self.outdir, self.label)
         self.sampling_time = datetime.timedelta()
-
-        signal.signal(signal.SIGTERM, self.write_current_state_and_exit)
-        signal.signal(signal.SIGINT, self.write_current_state_and_exit)
-        signal.signal(signal.SIGALRM, self.write_current_state_and_exit)
+        try:
+            signal.signal(signal.SIGTERM, self.write_current_state_and_exit)
+            signal.signal(signal.SIGINT, self.write_current_state_and_exit)
+            signal.signal(signal.SIGALRM, self.write_current_state_and_exit)
+        except AttributeError:
+            logger.info("Setting signal attributes unavailable on this system. "
+                        "This is not a problem if you don't work on an HPC system.")
 
     def __getstate__(self):
         """ For pickle: remove external_sampler, which can be an unpicklable "module" """