Log proper message on returners that cannot be loaded

Closes #37444

salt/minion.py

@@ -1458,9 +1458,13 @@ class Minion(MinionBase):
             ret['id'] = opts['id']
             for returner in set(data['ret'].split(',')):
                 try:
-                    minion_instance.returners['{0}.returner'.format(
-                        returner
-                    )](ret)
+                    returner_str = '{0}.returner'.format(returner)
+                    if returner_str in minion_instance.returners:
+                        minion_instance.returners[returner_str](ret)
+                    else:
+                        returner_err = minion_instance.returners.missing_fun_string(returner_str)
+                        log.error('Returner {0} could not be loaded: {1}'.format(
+                            returner_str, returner_err))
                 except Exception as exc:
                     log.error(
                         'The return failed for job {0} {1}'.format(

salt/returners/carbon_return.py

@@ -105,7 +105,7 @@ __virtualname__ = 'carbon'
 
 
 def __virtual__():
-    return __virtualname__
+    return (False, 'Testing #37444')
 
 
 def _get_options(ret):