Bug: New introduced DatabaseException not handled in BaseCRUDView

Current behaviour:

- If a DatabaseException is thrown, the UI shows an empty banner which has the same color like the background.

The DatabaseException should be catched like in api/\_\_init\_\_.py.
The message level should be set to "warning" like it was in 4.X.

```
try:
   ...
except DatabaseException as dex:
    flash(str(dex.__cause__), "warning")
```