fix not clear cause for invalid username

- add a message saying the reason for the value error exception
 - add a unit test to validate the expected message

Change-Id: I1d6cc0faa3a43852c46089e509d48cc3ee9f9cf8
Closes-Bug: #1911811

swift/common/middleware/tempauth.py

@@ -232,7 +232,11 @@ class TempAuth(object):
         self.users = {}
         for conf_key in conf:
             if conf_key.startswith(('user_', 'user64_')):
-                account, username = conf_key.split('_', 1)[1].split('_')
+                try:
+                    account, username = conf_key.split('_', 1)[1].split('_')
+                except ValueError:
+                    raise ValueError("key %s was provided in an "
+                                     "invalid format" % conf_key)
                 if conf_key.startswith('user64_'):
                     # Because trailing equal signs would screw up config file
                     # parsing, we auto-pad with '=' chars.

test/unit/common/middleware/test_tempauth.py

@@ -1246,6 +1246,14 @@ class TestParseUserCreation(unittest.TestCase):
             'user_admin_admin': 'admin .admin .reseller_admin',
         }), FakeApp())
 
+    def test_account_with_no_user(self):
+        expected_msg = 'key user_testtester was provided in an invalid format'
+        with self.assertRaises(ValueError) as ctx:
+            auth.filter_factory({
+                'user_testtester': 'testing',
+            })(FakeApp())
+        self.assertEqual(str(ctx.exception), expected_msg)
+
 
 class TestAccountAcls(unittest.TestCase):
     """