[Python] export_split_verilog can fail "silently"

The following code

```python
import circt

with circt.ir.Context() as ctx:
circt.register_dialects(ctx)
module = circt.ir.Module.parse("")
circt.export_split_verilog(module, "/bogusdirectory/")
```

prints

```
loc("-":0:0): error: cannot open output file '/bogusdirectory/filelist.f': No such file or directory
```

to the terminal, but doesn't result in a python-level exception. It would be good to be able to catch these at the python-level, ideally with an `MLIRError` like in other error cases.

Start with the Python reproduction using circt.export_split_verilog and inspect how output-file errors are surfaced. Confirm that passing /bogusdirectory/ raises a Python-level MLIRError rather than only printing the diagnostic.

Written by the indexing model from the issue text.