A successful UploadPart whose response carries no ETag header made
WriteChunk panic dereferencing uout.ETag in a debug log line. The part
ETag is required by CompleteMultipartUpload, so an ETag-less 200 is
unusable: return a retryable error from inside the pacer callback so
the chunk is retried instead of crashing the transfer or completing
the upload with a broken part list.
Fixes#9822
Co-authored-by: Shurong Cao <170531907+CAOShurong@users.noreply.github.com>
