XL: Change AppendFile() to return only error (#1932)

AppendFile ensures that it appends the entire buffer. Returns
an error otherwise, this patch removes the necessity for the
caller to look for 'n' return on short writes.

Ref #1893

erasure-createfile.go

@@ -137,15 +137,11 @@ func appendFile(disks []StorageAPI, volume, path string, enBlocks [][]byte, dist
 			defer wg.Done()
 			// Pick the block from the distribution.
 			blockIndex := distribution[index] - 1
-			n, wErr := disk.AppendFile(volume, path, enBlocks[blockIndex])
+			wErr := disk.AppendFile(volume, path, enBlocks[blockIndex])
 			if wErr != nil {
 				wErrs[index] = wErr
 				return
 			}
-			if n != int64(len(enBlocks[blockIndex])) {
-				wErrs[index] = errUnexpected
-				return
-			}
 
 			// Calculate hash for each blocks.
 			hashWriters[blockIndex].Write(enBlocks[blockIndex])