Replace `self.stop` with `Misc_Helpers.f_shutdown()`, and add it after existing `setverdict(fail, ...)` calls that previously fell through.
In all these cases there is no meaningful way to continue: the caller of these functions usually expects a valid value to be returned, and continuing (not stopping) oftentimes results in confusing DTEs.
s1gw: f_REST_*(): use tr_HTTP_RespBody(decmatch T:?)
Pass `exp := tr_HTTP_RespBody(decmatch T:?)` to f_http_transact() in every REST helper that decodes the response body. This ensures both a 2xx status code and a decodable body are required before any decode attempt is made, producing an informative "Unexpected HTTP response" verdict (rather than a confusing DTE) when e.g. an older osmo-s1gw returns HTTP 404 with an empty body for an unsupported endpoint.
