Rename argument msg to fail msg

This change make the arguments of assert less ambiguous.

lib/ansible/modules/utilities/logic/assert.py

@@ -27,9 +27,11 @@ options:
       - "A string expression of the same form that can be passed to the 'when' statement"
       - "Alternatively, a list of string expressions"
     required: true
-  msg:
+  fail_msg:
     description:
       - "The customized message used for a failing assertion"
+    aliases:
+      - msg
   success_msg:
     version_added: "2.7"
     description:
@@ -53,6 +55,6 @@ EXAMPLES = '''
     that:
       - "my_param <= 100"
       - "my_param >= 0"
-    msg: "'my_param' must be between 0 and 100"
+    fail_msg: "'my_param' must be between 0 and 100"
     success_msg: "'my_param' is between 0 and 100"
 '''

lib/ansible/plugins/action/assert.py

@@ -38,10 +38,13 @@ class ActionModule(ActionBase):
         if 'that' not in self._task.args:
             raise AnsibleError('conditional required in "that" string')
 
-        msg = None
+        fail_msg = None
         success_msg = None
-        if 'msg' in self._task.args and isinstance(self._task.args['msg'], string_types):
-            msg = self._task.args['msg']
+        if 'fail_msg' in self._task.args and isinstance(self._task.args['fail_msg'], string_types):
+            fail_msg = self._task.args['fail_msg']
+        elif 'msg' in self._task.args and isinstance(self._task.args['msg'], string_types):
+            fail_msg = self._task.args['msg']
+
         if 'success_msg' in self._task.args and isinstance(self._task.args['success_msg'], string_types):
             success_msg = self._task.args['success_msg']
 
@@ -65,8 +68,8 @@ class ActionModule(ActionBase):
                 result['evaluated_to'] = test_result
                 result['assertion'] = that
 
-                if msg:
-                    result['msg'] = msg
+                if fail_msg:
+                    result['msg'] = fail_msg
 
                 return result