Use raise ... from e instead of raise ... only when catching error with try-except in python

[TommyDew42]

What language does this apply to?
Python

Describe the problem you are trying to solve.
In some of the python code, we only raise an error without using the from after catching an error with except.
For example: json_format.py
This makes debugging sometimes difficult, especially the try block is a huge chunk of logic. We are not sure which line of code caused the error.

Describe the solution you'd like
Use raise ... from ... instead of raise ... only

Describe alternatives you've considered

Additional context
Happy to work on this if we think it's a good idea.

[acozzette]

This sounds like a reasonable change and we would be happy to have someone review it if you would like to send a pull request.