tsweb: when unwrapping HTTPError, record the user-facing message also in the log

There's often some useful piece of information in there not already
repeated in the internal error.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

tsweb/tsweb.go

@@ -236,8 +236,13 @@ func (h retHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	case hErrOK:
 		// Handler asked us to send an error. Do so, if we haven't
 		// already sent a response.
+		msg.Err = hErr.Msg
 		if hErr.Err != nil {
-			msg.Err = hErr.Err.Error()
+			if msg.Err == "" {
+				msg.Err = hErr.Err.Error()
+			} else {
+				msg.Err = msg.Err + ": " + hErr.Err.Error()
+			}
 		}
 		if lw.code != 0 {
 			h.logf("[unexpected] handler returned HTTPError %v, but already sent a response with code %d", hErr, lw.code)