net/http: add proper panic message

Change-Id: Ibfb330eaf24e004ddec60a5ca08cdc780235ad8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/688315
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Mark Freeman <mark@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/net/http/server.go

@@ -1614,7 +1614,7 @@ func writeStatusLine(bw *bufio.Writer, is11 bool, code int, scratch []byte) {
 // It's illegal to call this before the header has been flushed.
 func (w *response) bodyAllowed() bool {
 	if !w.wroteHeader {
-		panic("")
+		panic("net/http: bodyAllowed called before the header was written")
 	}
 	return bodyAllowedForStatus(w.status)
 }