lib/rest: validate ranged responses

Add response validation for calls made with Range open options. Verify
Content-Range, Content-Length, response status, and the complete
representation size before a backend accepts the response body.

Return a shared sentinel when a server ignores a partial range so callers
can avoid retrying the same unsupported request.

fs/fs.go

@@ -53,6 +53,7 @@ var (
 	ErrorFileNameTooLong             = errors.New("file name too long")
 	ErrorCantListRoot                = errors.New("can't list root")
 	ErrorFileTooSmall                = errors.New("file too small for multipart upload")
+	ErrorRangeIgnored                = errors.New("server ignored requested range")
 )
 
 // FileTooSmallError is returned by OpenChunkWriter when a file is below the