Default error messages not getting overridden

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I'm trying to add a customized error message for a class field in Marshamallow's schema. The field representation looks like this:

```
phone_number = fields.String(validate=Length(max=20), error_messages={'invalid': 'Phone number must be a string shorter than 20 letters.'})
```

When loading the request data to a schema with a longer phone number like:

```
try:
request_data = EditInvestorSchema().load(request.json)
except ValidationError as error:
return get_response(400, list(error.messages.values())[0])
```

The default error message "Longer than maximum length 20." arises instead of the customized error message.

Start by reproducing the schema load using fields.String, Length(max=20), and the custom error_messages shown in the issue. Inspect how validation errors from Length are converted into ValidationError messages; done means an overlong phone number produces the customized message instead of the default.

Written by the indexing model from the issue text.