fix(regtest): prevent nil-response panic during block retrieval

The regression runner could panic while fetching /thorchain/block because it accessed res.StatusCode before checking the error returned by httpClient.Get(). Failed requests can return a nil response, causing the runner to crash and hide the underlying HTTP error. During local regression runs, this happened three times. A resumed run passed that suite but crashed again after 27 of ~150 tests completed. Both incidents pointed to OpCreateBlocks.Execute in operations.go:621, and the command exited with code 2. The underlying HTTP failure could not be determined from the panic logs.

This change checks request errors before accessing the response, retains the existing five-attempt retry limit, and closes unsuccessful response bodies before retrying. Persistent failures reach the existing error-reporting path instead of causing a nil-pointer panic.