Improve celery upgrade error handling (#4825)

CONTRIBUTORS.txt

@@ -260,4 +260,5 @@ Igor Kasianov, 2018/01/20
 Derek Harland, 2018/02/15
 Chris Mitchell, 2018/02/27
 Josue Balandrano Coronel, 2018/05/24
+Federico Bond, 2018/06/20
 Tom Booth, 2018/07/06

celery/bin/upgrade.py

@@ -41,8 +41,12 @@ class upgrade(Command):
             raise self.UsageError('unknown upgrade type: {0}'.format(command))
         return getattr(self, command)(*args, **kwargs)
 
-    def settings(self, command, filename,
+    def settings(self, command, filename=None,
                  no_backup=False, django=False, compat=False, **kwargs):
+
+        if filename is None:
+            raise self.UsageError('missing settings filename to upgrade')
+
         lines = self._slurp(filename)
         keyfilter = self._compat_key if django or compat else pass1
         print('processing {0}...'.format(filename), file=self.stderr)

t/unit/bin/test_upgrade.py

@@ -0,0 +1,22 @@
+# -*- coding: utf-8 -*-
+"""Tests for ``celery upgrade`` command."""
+from __future__ import absolute_import, unicode_literals
+
+import pytest
+
+from celery.bin.celery import upgrade
+from celery.five import WhateverIO
+
+
+class test_upgrade:
+    """Test upgrade command class."""
+
+    def test_run(self):
+        out = WhateverIO()
+        a = upgrade(app=self.app, stdout=out)
+
+        with pytest.raises(a.UsageError, match=r'missing upgrade type'):
+            a.run()
+
+        with pytest.raises(a.UsageError, match=r'missing settings filename'):
+            a.run('settings')