Improve error handling for render_template() in topgeb

Topgen uses helper functions called `render_template` like the one below:

```python
# Template render helper
def render_template(template_path: str, rendered_path: Path, **other_info):
template_contents = generate_top(completecfg, name_to_block,
str(template_path), **other_info)

rendered_path.parent.mkdir(exist_ok=True, parents=True)
with rendered_path.open(mode="w", encoding="UTF-8") as fout:
fout.write(template_contents)
```

These functions miss proper error handling if the `open` syscall fails or `generate_top` returns an empty string.

/cc @matutem

Find the topgen implementation of render_template() and inspect how generate_top() results and rendered_path.open() failures are currently handled. Reproduce the empty-output and open-syscall failure cases, then add the requested error handling and verify the relevant topgen checks or tests if present.

Written by the indexing model from the issue text.