Django ValidationError are serialized as string instead of error array

* **What is the current behavior?**

Django ValidationError are serialized as string instead of error array.

```python
from django.core.exceptions import ValidationError

from graphene import ObjectType, Schema, Int

class Query(ObjectType):
test = Int()

def resolve_test(root, context, **kwargs):
raise ValidationError("error message")

schema = Schema(query=Query)
```
or [https://repl.it/repls/AffectionateObviousBookmark#cookbook/schema.py](https://repl.it/repls/AffectionateObviousBookmark#cookbook/schema.py)

For the following query:
```
query Test {
test
}
```

We got the following response:
```
{
"errors": [
{
"message": "['error message']",
"locations": [
{
"line": 33,
"column": 3
}
],
"path": [
"test"
]
}
],
"data": {
"test": null
}
}
```
Look at the message field ...

* **What is the expected behavior?**

```
{
"errors": [
{
"message": "error message",
"locations": [
{
"line": 33,
"column": 3
}
],
"path": [
"test"
]
}
],
"data": {
"test": null
}
}
```

* **Please tell us about your environment:**

- Version: django (^3.1), django-graphene (^2.13)
- Platform: linux

* **Other information** (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow)