nbdev_docs doesn't provide location of error

Hi,
I noticed that nbdev_docs doesn't provide location where the error occured, if there is no heading in the ipynb file.

**With heading:**
```
Exception: Error in notebook: utils_files in cell 3 :
r = 1/0
import sys
```

**Without heading:**
```
Exception: Error in cell 2 :
r = 1/0
import sys
```

Is it possible somehow fix it and add file name into the Exception info?

Start by tracing nbdev_docs handling of errors from ipynb notebooks, comparing the reported output for notebooks with and without headings. Confirm how the notebook name and cell number are currently selected. Done means the exception includes the notebook file name in both cases.

Written by the indexing model from the issue text.