storage/app: handle upload errors consistently

Change-Id: I79f486a875e4c6858482d406b1c0328655e32aef
Reviewed-on: https://go-review.googlesource.com/38305
Run-TryBot: Quentin Smith <quentin@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

storage/app/upload.go

@@ -33,7 +33,9 @@ func (a *App) upload(w http.ResponseWriter, r *http.Request) {
 	case err == ErrResponseWritten:
 		return
 	case err != nil:
+		errorf(ctx, "%v", err)
 		http.Error(w, err.Error(), 500)
+		return
 	}
 
 	if r.Method == http.MethodGet {
@@ -92,6 +94,9 @@ func (a *App) processUpload(ctx context.Context, user string, mr *multipart.Read
 		if err == io.EOF {
 			break
 		}
+		if err != nil {
+			return nil, err
+		}
 
 		name := p.FormName()
 		if name == "commit" {