fix: return bad request on malformed packages upload input (#10954)

I noticed that the wrong content type in an `/upload` request can trigger a 500, and I'm guessing it is more appropriate to return 400 instead.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/10954
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Roberto Vidal <roberto.vidal@ikumene.com>
Co-committed-by: Roberto Vidal <roberto.vidal@ikumene.com>

services/context/context_request.go

@@ -30,3 +30,7 @@ func (ctx *Context) UploadStream() (rd io.ReadCloser, needToClose bool, err erro
 	}
 	return ctx.Req.Body, false, nil
 }
+
+func IsFormError(err error) bool {
+	return err == http.ErrMissingFile || err == http.ErrMissingBoundary
+}