cmd/zoekt-webserver: abort watchdog with log.Panicf

Fixes #34.

Change-Id: I4b8fc35956c2aee44458065b96f357f5a6836dd8
diff --git a/cmd/zoekt-webserver/main.go b/cmd/zoekt-webserver/main.go
index 2533a44..f7433f3 100644
--- a/cmd/zoekt-webserver/main.go
+++ b/cmd/zoekt-webserver/main.go
@@ -236,7 +236,7 @@
 	for _ = range tick.C {
 		err := watchdogOnce(context.Background(), client, addr)
 		if err != nil {
-			log.Fatalf("watchdog: %v", err)
+			log.Panicf("watchdog: %v", err)
 		}
 	}
 }