mime/multipart: use %w when wrapping error in NextPart

Use "%w" instead of "%v" as format verb for error value in the NextPart
method. This way it will be possible to use common go error utilities
from std library when parsing from custom io.Readers.

This issue was discovered during attempts to use
http.Request.ParseMultipartForm together with http.MaxBytesHandler.

Change-Id: Idb82510fb536b66b51ed1d943737c4828f07c2f2
GitHub-Last-Rev: 8bc49c945c
GitHub-Pull-Request: golang/go#55133
Reviewed-on: https://go-review.googlesource.com/c/go/+/431675
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

src/mime/multipart/multipart.go

@@ -360,7 +360,7 @@ func (r *Reader) nextPart(rawPart bool) (*Part, error) {
 			return nil, io.EOF
 		}
 		if err != nil {
-			return nil, fmt.Errorf("multipart: NextPart: %v", err)
+			return nil, fmt.Errorf("multipart: NextPart: %w", err)
 		}
 
 		if r.isBoundaryDelimiterLine(line) {