Catches SystemExit in case Pymultinest is not installed

bilby/core/sampler/base_sampler.py

@@ -139,7 +139,7 @@ class Sampler(object):
         external_sampler_name = self.__class__.__name__.lower()
         try:
             self.external_sampler = __import__(external_sampler_name)
-        except ImportError:
+        except (ImportError, SystemExit):
             raise ImportError(
                 "Sampler {} not installed on this system".format(external_sampler_name))