Be more resilient in the case of missing omniauth settings

Should no longer freak out when omniauth settings aren't present in
gitlab.yml. People who aren't using it shouldn't even have to put a
'false' entry in their config for it (and probably wouldn't, after an
upgrade).

config/initializers/1_settings.rb

@@ -121,19 +121,19 @@ class Settings < Settingslogic
     end
 
     def ldap_enabled?
-      ldap['enabled']
-    rescue
+      ldap && ldap['enabled']
+    rescue Settingslogic::MissingSetting
       false
     end
 
     def omniauth_enabled?
       omniauth && omniauth['enabled']
-    rescue
+    rescue Settingslogic::MissingSetting
       false
     end
 
     def omniauth_providers
-      omniauth['providers'] || []
+      (omniauth_enabled? && omniauth['providers']) || []
     end
 
     def disable_gravatar?