Display proper error on group_vars syntax error

This fix ensures a proper error is shown when a group_vars files cannot
be parsed correctly. Without this patch you get:

```
[dag@moria ansible.testing]$ ansible-playbook test132.yml
ERROR! Unexpected Exception: dictionary update sequence element #0 has length 1; 2 is required
to see the full traceback, use -vvv
```

With this patch you get:
```
[dag@moria ansible.testing]$ ansible-playbook test132.yml
ERROR! Problem parsing file '/home/dag/home-made/ansible.testing/group_vars/test135': line 1, column 1
```

This fixes #18843

lib/ansible/vars/__init__.py

@@ -583,8 +583,13 @@ class VariableManager:
 
         rval = AnsibleInventoryVarsData()
         rval.path = path
+
         if data is not None:
+            if not isinstance(data, dict):
+                raise AnsibleError("Problem parsing file '%s': line %d, column %d" % data.ansible_pos)
+            else:
                 rval.update(data)
+
         return rval
 
     def add_host_vars_file(self, path, loader):