Need more debugging information in case if notebook parsing fails

When running `nbdev_trust` I get a long stack trace which ends with
```
File "/home/mib07150/miniconda3/envs/py38/lib/python3.8/json/decoder.py", line 353, in raw_decode
obj, end = self.scan_once(s, idx)
json.decoder.JSONDecodeError: Expecting ',' delimiter: line 211 column 4 (char 7193)
```

However, there is more than 30 notebooks in this folder, and there is no sign of which of these is the problematic one -- nor what to do to make it parse-able again

I think that there might be an incompatibility between `plotly` and the `json` parser used by `nbdev`

I will not submit the problematic notebook, because I don't even know which one is it, nor which section of it

desirable info:
-> Notebook name
-> Section or cell number
-> surrounding "user-readable text" so that the offending section can be found easily

The entry point is the `nbdev_trust` command; begin by tracing its notebook-parsing error path and how JSONDecodeError is reported. Done means a parsing failure identifies the notebook, section or cell, and nearby user-readable text so the offending content can be found and corrected.

Written by the indexing model from the issue text.