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?