Resolve little typing issue in case of IndexError:

```
  File "/usr/lib/python3.8/site-packages/rkd/yaml_parser.py", line 64, in load
    raise YAMLFileValidationError(e)
  File "/usr/lib/python3.8/site-packages/rkd/exception.py", line 51, in __init__
    '.'.join(err.path), err.message
TypeError: sequence item 2: expected str instance, int found
```

src/rkd/exception.py

@@ -48,7 +48,7 @@ class YAMLFileValidationError(YamlParsingException):
 
     def __init__(self, err: ValidationError):
         super().__init__('YAML schema validation failed at path "%s" with error: %s' % (
-            '.'.join(err.path), str(err.message)
+            str(list(err.path)), str(err.message)
         ))