API: add writePartTooSmallErrorResponse to extend standard error responses. (#2005)

This function is added to extend the standard error responses.
Which is needed in some cases for example CompleteMultipartUpload
should respond with ErrPartTooSmall error when parts uploaded are
lesser than 5MB (i.e minimum allowed size per part).

Fixes #1536

fs-v1-multipart.go

@@ -504,7 +504,11 @@ func (fs fsObjects) CompleteMultipartUpload(bucket string, object string, upload
 		}
 		// All parts except the last part has to be atleast 5MB.
 		if (i < len(parts)-1) && !isMinAllowedPartSize(fsMeta.Parts[partIdx].Size) {
-			return "", PartTooSmall{}
+			return "", PartTooSmall{
+				PartNumber: part.PartNumber,
+				PartSize:   fsMeta.Parts[partIdx].Size,
+				PartETag:   part.ETag,
+			}
 		}
 		// Construct part suffix.
 		partSuffix := fmt.Sprintf("object%d", part.PartNumber)