more robust settings in pre-merge phase

when we haven't migrated yet, there is no settings table.
this doesn't hinder plugins to read/write settings (when
initializing the app).

solution: ignore setting-writes (but give a warning) and
answer an empty hash on setting-read when there is no
setting-table.

fixes #1830

app/models/setting.rb

@@ -81,15 +81,28 @@ class Setting < ActiveRecord::Base
     # or set using Setting.some_setting_name = "some value"
     src = <<-END_SRC
       def self.#{name}
+        if connection.table_exists?(table_name)
           self[:#{name}]
+        else
+          {} # when runnung too early, there is no settings table. do nothing
+        end
       end
 
       def self.#{name}?
+        if connection.table_exists?(table_name)
           self[:#{name}].to_i > 0
+        else
+          {} # when runnung too early, there is no settings table. do nothing
+        end
       end
 
       def self.#{name}=(value)
+        if connection.table_exists?(table_name)
           self[:#{name}] = value
+        else
+          logger.warn "Trying to save a setting named '#{name}' while there is no 'setting' table yet. This setting will not be saved!"
+          {} # when runnung too early, there is no settings table. do nothing
+        end
       end
     END_SRC
     class_eval src, __FILE__, __LINE__