Integrity errors not handle by REST API

[vchendrix]

Database integrity errors are returned as INternal server errors (500). They should have a nicer handling.

(.venv)vagrant@ubuntu-14:/vagrant$ tail -100f ngt-archive.log 
    response = self._get_response(request)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/core/handlers/base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/views/decorators/csrf.py", line 58, in wrapped_view
    return view_func(*args, **kwargs)
  File "/vagrant/.venv/lib/python3.4/site-packages/rest_framework/viewsets.py", line 83, in view
    return self.dispatch(request, *args, **kwargs)
  File "/vagrant/.venv/lib/python3.4/site-packages/rest_framework/views.py", line 477, in dispatch
    response = self.handle_exception(exc)
  File "/vagrant/.venv/lib/python3.4/site-packages/rest_framework/views.py", line 437, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/vagrant/.venv/lib/python3.4/site-packages/rest_framework/views.py", line 474, in dispatch
    response = handler(request, *args, **kwargs)
  File "/vagrant/.venv/lib/python3.4/site-packages/rest_framework/mixins.py", line 21, in create
    self.perform_create(serializer)
  File "./archive_api/viewsets.py", line 68, in perform_create
    serializer.save(created_by=self.request.user, modified_by=self.request.user)
  File "/vagrant/.venv/lib/python3.4/site-packages/rest_framework/serializers.py", line 214, in save
    self.instance = self.create(validated_data)
  File "./archive_api/serializers.py", line 141, in create
    dataset = DataSet.objects.create(**validated_data)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/db/models/query.py", line 399, in create
    obj.save(force_insert=True, using=self.db)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/db/models/base.py", line 796, in save
    force_update=force_update, update_fields=update_fields)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/db/models/base.py", line 824, in save_base
    updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/db/models/base.py", line 908, in _save_table
    result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/db/models/base.py", line 947, in _do_insert
    using=using, raw=raw)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/db/models/query.py", line 1045, in _insert
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/db/models/sql/compiler.py", line 1054, in execute_sql
    cursor.execute(sql, params)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/db/backends/utils.py", line 79, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/db/utils.py", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/utils/six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "/vagrant/.venv/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
django.db.utils.IntegrityError: duplicate key value violates unique constraint "archive_api_dataset_pkey"
DETAIL:  Key (id)=(6) already exists.