Exception.message was deprecated in Python 2.6 and removed in Python 3

`BaseException.message` was deprecated as of Python 2.6 and is removed in Python 3. Use `str(e)` to access the user-readable message. Use `e.args` to access arguments passed to the exception.

$ `python3`
```

>>> dir(Exception('Hey'))
['__cause__', '__class__', '__context__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setstate__', '__sizeof__', '__str__', '__subclasshook__', '__suppress_context__', '__traceback__', 'args', 'with_traceback']
>>> try:
...     blah
... except Exception as e:
...     print(e)
...     print(e.args)
...     print(e.message)  # Will cause a second Exception
...     
name 'blah' is not defined
("name 'blah' is not defined",)
Traceback (most recent call last):
  File "<string>", line 2, in <module>
NameError: name 'blah' is not defined

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<string>", line 6, in <module>
AttributeError: 'NameError' object has no attribute 'message'
```

netbox/netbox/models.py

@@ -115,7 +115,7 @@ class CustomFieldsMixin(models.Model):
             try:
                 custom_fields[field_name].validate(value)
             except ValidationError as e:
-                raise ValidationError(f"Invalid value for custom field '{field_name}': {e.message}")
+                raise ValidationError(f"Invalid value for custom field '{field_name}': {e}")
 
         # Check for missing required values
         for cf in custom_fields.values():