Allow the msg argument to fail_json() to be a positional argument

fial_json() requires a message be given to it to inform the end user of
why the module failed.  Prior to this commit, the message had to be a
keyword argument:

    module.fail_json(msg='Failed due to error')

Since this is a required parameter, this commit allows the message to be
given as a positional argument instead:

   module.fail_json('Failed due to an error')

changelogs/fragments/allow-fail-json-msg-to-be-positional.yaml

@@ -0,0 +1,5 @@
+minor_features:
+  - `AnsibleModule.fail_json()` has always required that a message be passed
+    in which informs the end user why the module failed.  In the past this
+    message had to be passed as the `msg` keyword argument but it can now be
+    passed as the first positional argument instead.

lib/ansible/module_utils/basic.py

@@ -2041,12 +2041,11 @@ class AnsibleModule(object):
         self._return_formatted(kwargs)
         sys.exit(0)
 
-    def fail_json(self, **kwargs):
+    def fail_json(self, msg, **kwargs):
         ''' return from the module, with an error message '''
 
-        if 'msg' not in kwargs:
-            raise AssertionError("implementation error -- msg to explain the error is required")
         kwargs['failed'] = True
+        kwargs['msg'] = msg
 
         # Add traceback if debug or high verbosity and it is missing
         # NOTE: Badly named as exception, it really always has been a traceback

test/units/module_utils/basic/test_exit_json.py

@@ -55,11 +55,23 @@ class TestAnsibleModuleExitJson:
         expected['failed'] = True
         assert return_val == expected
 
+    @pytest.mark.parametrize('stdin', [{}], indirect=['stdin'])
+    def test_fail_json_msg_positional(self, am, capfd):
+        with pytest.raises(SystemExit) as ctx:
+            am.fail_json('This is the msg')
+        assert ctx.value.code == 1
+
+        out, err = capfd.readouterr()
+        return_val = json.loads(out)
+        # Fail_json should add failed=True
+        assert return_val == {'msg': 'This is the msg', 'failed': True,
+                              'invocation': EMPTY_INVOCATION}
+
     @pytest.mark.parametrize('stdin', [{}], indirect=['stdin'])
     def test_fail_json_no_msg(self, am):
-        with pytest.raises(AssertionError) as ctx:
+        with pytest.raises(TypeError) as ctx:
             am.fail_json()
-        assert ctx.value.args[0] == "implementation error -- msg to explain the error is required"
+        assert ctx.value.args[0] == "fail_json() missing 1 required positional argument: 'msg'"
 
 
 class TestAnsibleModuleExitValuesRemoved: