Enable callstack in 400 response, if STRIPE_LOG=debug

[lowlyocean]

Resolves #155

[adrienverge]

Thanks for the proposal.
I don't see the usefulness of such an addition, that would moreover leak information about the host system + Python paths. I believe it was useful for you when you debugged #151? But won't be useful for localstripe users when they use the server.
What do you think?

[lowlyocean]

Yes I was thinking about the potential to leak, it's a valid concern. However the approach here is to require an environment variable to be purposefully enabled in to activate the behavior. My use case is a Gitlab CI pipeline which spawns localstripe as a Docker container, and then my existing end-to-end tests are run against it. It's quite helpful in the CI logs to see why Stripe requests failed.