Prevent 500 error on ldap group links page

If group has links to ldap servers that are disabled or removed from
gitlab config.
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/ldap_group_link.rb

@@ -14,6 +14,8 @@ class LdapGroupLink < ActiveRecord::Base
 
   def config
     Gitlab::LDAP::Config.new(provider)
+  rescue Gitlab::LDAP::Config::InvalidProvider
+    nil
   end
 
   # default to the first LDAP server

app/views/ldap_group_links/_ldap_group_link.html.haml

 %li
-  = ldap_group_link.cn
-  %small.light== as #{ldap_group_link.human_access} on #{ldap_group_link.provider_label}
+  %h4= ldap_group_link.cn
   .pull-right
     = link_to group_ldap_group_link_path(group, ldap_group_link), method: :delete, class: 'btn btn-danger btn-small' do
       = fa_icon('unlink', text: 'unlink')
+
+  - if ldap_group_link.config
+    %p.light
+      As
+      %strong #{ldap_group_link.human_access}
+      on
+      %strong #{ldap_group_link.provider_label}
+      server
+  - else
+    %p.cred
+      %i.fa.fa-warning
+      Config for
+      %code #{ldap_group_link.provider}
+      does not present in GitLab

lib/gitlab/ldap/config.rb

@@ -4,12 +4,16 @@ module Gitlab
     class Config
       attr_accessor :provider, :options
 
+      class InvalidProvider < StandardError; end
+
       def self.enabled?
         Gitlab.config.ldap.enabled
       end
 
       def self.servers
         Gitlab.config.ldap.servers.values
+      rescue Settingslogic::MissingSetting
+        []
       end
 
       def self.providers
@@ -21,7 +25,7 @@ module Gitlab
       end
 
       def self.invalid_provider(provider)
-        raise "Unknown provider (#{provider}). Available providers: #{providers}"
+        raise InvalidProvider.new("Unknown provider (#{provider}). Available providers: #{providers}")
       end
 
       def initialize(provider)
@@ -32,6 +36,7 @@ module Gitlab
         else
           self.class.invalid_provider(provider)
         end
+
         @options = config_for(@provider) # Use @provider, not provider
       end
 
@@ -87,6 +92,7 @@ module Gitlab
       end
 
       protected
+
       def base_config
         Gitlab.config.ldap
       end