Better error message in `_prepare_output_docstrings`
# What does this PR do?

Currently, if an output type has no docstring, or its docstring doesn't have `Args` or `Parameters`, we get an error 
```bash
  File "/transformers/src/transformers/utils/doc.py", line 137, in _prepare_output_docstrings
    full_output_type = f"{output_type.__module__}.{output_type.__name__}"
UnboundLocalError: local variable 'params_docstring' referenced before assignment
```
when `_prepare_output_docstrings` is called (for example, running a script that uses the relevant model).

This is not super informative what's wrong and what to fix.

This PR adds an error message to explain what's going on.
_The documentation is not available anymore as the PR was closed or merged._