Add exception for PermissionError

This happens when there is no permission to execute the external program
or the external program does not exist.

ospd/ospd.py

@@ -1051,7 +1051,7 @@ def handle_get_performance(self, scan_et):
 
         try:
             output = subprocess.check_output(cmd)
-        except subprocess.CalledProcessError as e:
+        except (subprocess.CalledProcessError, PermissionError) as e:
             raise OspdCommandError(
                 'Bogus get_performance format. %s' % e,
                 'get_performance'