Minor fixes to the logger output

1. Print message when generating live points. This helps diagnose cases where the generation takes a long time.
2. Downgrade warning message to debug for returning random point.

For 2, we have seen it hitting this message quite often during runs, however it doesn't appear to cause substantial issues. It can be diagnosed from the run plots. Meanwhile, the message itself is dominating run output.

bilby/core/sampler/base_sampler.py

@@ -430,6 +430,7 @@ class Sampler(object):
             likelihood evaluations.
 
         """
+        logger.info("Generating initial points from the prior")
         unit_cube = []
         parameters = []
         likelihood = []