fix bootup with specified redis database

(was making a call before AR connected)

catch the failure in Setting.get and return the default
but don't cache it

Change-Id: I6a184a818f6ec12e036a468d31c5d8ebfae3068c
Reviewed-on: https://gerrit.instructure.com/109703
Tested-by: Jenkins
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>

app/models/setting.rb

@@ -28,11 +28,12 @@ class Setting < ActiveRecord::Base
     else
       begin
         from_db = Setting.where(name: name).first.try(:value)
-      rescue ActiveRecord::StatementInvalid => e
+        @@cache[name] = from_db || default.try(:to_s)
+      rescue ActiveRecord::StatementInvalid, ActiveRecord::ConnectionNotEstablished => e
         # the db may not exist yet
-        Rails.logger.warn("Unable to read setting: #{e}")
+        Rails.logger.warn("Unable to read setting: #{e}") if Rails.logger
+        default.try(:to_s)
       end
-      @@cache[name] = from_db || default.try(:to_s)
     end
   end