(chore): add error string to handle_snapshot

[tabgok]

Prior to this change debugging the handle_snapshot function was difficult, because no error message beyond "INTERNAL SERVER ERROR" was returned. After this change, the exception's message is returned.

[Kyle-Verhoog]

Oh this is a nice usability improvement. I think we can implement this generally in a middleware rather than just for this one endpoint.

Here's an example middleware that we already have

dd-apm-test-agent/ddapm_test_agent/agent.py

Lines 99 to 107 in e0eb48d

	@middleware
	async def session_token_middleware(request: Request, handler: _Handler) -> web.Response:
	"""Extract session token from the request and store it in the request.
	The token is retrieved from the headers or params of the request.
	"""
	token = _session_token(request)
	request["session_token"] = token
	return await handler(request)

Could you check to see if we could do that approach so we have this for all endpoints, @tabgok?

[Kyle-Verhoog]

nice!