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

object-errors.go

@@ -251,8 +251,12 @@ func (e InvalidPartOrder) Error() string {
 }
 
 // PartTooSmall - error if part size is less than 5MB.
-type PartTooSmall struct{}
+type PartTooSmall struct {
+	PartSize   int64
+	PartNumber int
+	PartETag   string
+}
 
 func (e PartTooSmall) Error() string {
-	return "Part size should be atleast 5MB"
+	return fmt.Sprintf("Part size for %d should be atleast 5MB", e.PartNumber)
 }