[hooks] do not crash on startup if the config entry is missing

hooks.py

@@ -23,12 +23,13 @@ class ServerStartupHook(hook.Hook):
         self.debug('registering kerberos authentifier')
         self.repo.system_source.add_authentifier(XRemoteUserAuthentifier())
 
+CONFENTRY = 'secret-key-filepath'
 def registration_callback(vreg):
-    vreg.register(ServerStartupHook)
-    fpath = vreg.config.get('secret-key-filepath')
+    fpath = vreg.config.get(CONFENTRY)
     if fpath is None or not osp.exists(fpath):
-        msg = 'invalid or missing value for the secret-key-filepth'
+        msg = 'invalid or missing value for the %s' % CONFENTRY
         vreg.error(msg)
-        raise Exception(msg)
-    vreg.config._secret = open(fpath, 'rb').read().strip()
-    vreg.debug('loaded secret key')
+    else:
+        vreg.register(ServerStartupHook)
+        vreg.config._secret = open(fpath, 'rb').read().strip()
+        vreg.debug('loaded secret key')