fix: put an error message for deleting wo dictionaries

[skygus]

I ran into an issue trying to delete a write-only dictionary, and it was difficult to understand what was happening. It returns an error: Bad request: Not allowed to read contents of write_only dictionary, which doesn't make any sense, since I'm trying to delete it.

I see there's a check for deleting a dictionary which lists the contents, but this won't work for write only dictionaries, so we should have a check to see if it's a write only dictionary first, and return an error to say you should have the force_destroy flag.

TBH, I didn't understand how to add a test for this, so this PR is basically informational.