don't return dev messages in getUserMessage()

[xavdid-stripe]

Why?

Per guidance in V2 error guidelines, the user_message response field can be shown directly to users, but the message field is meant only for developers. The previous java implementation (#1911) conflated the two, which I worry may be confusing for users. I think the intended use case here is someting like:

1. stripe error happened!
2. if .getUserMessage() != null, return that to the user that triggered this action
3. otherwise, check which error class it is and handle/log accordingly

What?

• return null from getUserMessage() for v1 errors
• tweak docstrings
• simplify version checking logic
• update test

See Also

• DEVSDK-2262
• V2 error guidelines

Changelog

• stripeException.getUserMessage() now returns null for v1 errors instead of the developer-facing message
  • getUserMessage(), if non-null, can be passed directly to your users to explain what went wrong
  • if getUserMessage() is null, then you should handle the error as usual, per the error handling docs.

[xavdid-stripe]

Tabling for now, but we'll revisit in the future.