Event.full_clean() raises non-ValidationError exceptions

This leads to server errors in the admin. These should be converted to `ValidationError`.

https://docs.djangoproject.com/en/2.0/ref/models/instances/#django.db.models.Model.full_clean

```
Traceback:

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/core/handlers/exception.py" in inner
35. response = get_response(request)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
128. response = self.process_exception_by_middleware(e, request)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
126. response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/contrib/admin/options.py" in wrapper
575. return self.admin_site.admin_view(view)(*args, **kwargs)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/utils/decorators.py" in _wrapped_view
142. response = view_func(request, *args, **kwargs)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/views/decorators/cache.py" in
_wrapped_view_func
44. response = view_func(request, *args, **kwargs)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/contrib/admin/sites.py" in inner
223. return view(request, *args, **kwargs)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/contrib/admin/options.py" in add_view
1554. return self.changeform_view(request, None, form_url, extra_context)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/utils/decorators.py" in _wrapper
62. return bound_func(*args, **kwargs)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/utils/decorators.py" in _wrapped_view
142. response = view_func(request, *args, **kwargs)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/utils/decorators.py" in bound_func
58. return func.__get__(self, type(self))(*args2, **kwargs2)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/contrib/admin/options.py" in changeform_view
1451. return self._changeform_view(request, object_id, form_url, extra_context)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/contrib/admin/options.py" in
_changeform_view
1483. if form.is_valid():

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/forms/forms.py" in is_valid
179. return self.is_bound and not self.errors

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/forms/forms.py" in errors
174. self.full_clean()

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/forms/forms.py" in full_clean
378. self._post_clean()

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/forms/models.py" in _post_clean
401. self.instance.full_clean(exclude=exclude, validate_unique=False)

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/db/models/base.py" in full_clean
1151. self.clean()

File "./course/models.py" in clean
342. course=self.course,

File "/home/andreas/my-relate-env3/lib/python3.6/site-packages/django/db/models/fields/related_descriptors.py" in
__get__
175. "%s has no %s." % (self.field.model.__name__, self.field.name)

Exception Type: RelatedObjectDoesNotExist at /admin/course/event/add/
Exception Value: Event has no course.
Request information:
```