Fixed 'raw' raises by converting them to the appropriate OSM::APIError type. Made the error messages for placeholder fixing more informative. Added tests for these.

app/models/node.rb

@@ -267,7 +267,7 @@ class Node < ActiveRecord::Base
   ##
   # dummy method to make the interfaces of node, way and relation
   # more consistent.
-  def fix_placeholders!(id_map)
+  def fix_placeholders!(id_map, placeholder_id = nil)
     # nodes don't refer to anything, so there is nothing to do here
   end
   

app/models/relation.rb

@@ -318,12 +318,12 @@ class Relation < ActiveRecord::Base
   # if any members are referenced by placeholder IDs (i.e: negative) then
   # this calling this method will fix them using the map from placeholders 
   # to IDs +id_map+. 
-  def fix_placeholders!(id_map)
+  def fix_placeholders!(id_map, placeholder_id = nil)
     self.members.map! do |type, id, role|
       old_id = id.to_i
       if old_id < 0
         new_id = id_map[type.downcase.to_sym][old_id]
-        raise "invalid placeholder" if new_id.nil?
+        raise OSM::APIBadUserInput.new("Placeholder #{type} not found for reference #{old_id} in relation #{self.id.nil? ? placeholder_id : self.id}.") if new_id.nil?
         [type, new_id, role]
       else
         [type, id, role]

app/models/way.rb

@@ -287,11 +287,11 @@ class Way < ActiveRecord::Base
   # if any referenced nodes are placeholder IDs (i.e: are negative) then
   # this calling this method will fix them using the map from placeholders 
   # to IDs +id_map+. 
-  def fix_placeholders!(id_map)
+  def fix_placeholders!(id_map, placeholder_id = nil)
     self.nds.map! do |node_id|
       if node_id < 0
         new_id = id_map[:node][node_id]
-        raise "invalid placeholder for #{node_id.inspect}: #{new_id.inspect}" if new_id.nil?
+        raise OSM::APIBadUserInput.new("Placeholder node not found for reference #{node_id} in way #{self.id.nil? ? placeholder_id : self.id}") if new_id.nil?
         new_id
       else
         node_id