pass common API error text through to Potlatch so it can be shown to the user

lib/osm.rb

@@ -13,6 +13,10 @@ module OSM
     def render_opts
       { :text => "Generic API Error", :status => :internal_server_error, :content_type => "text/plain" }
     end
+
+    def to_s
+      "Generic API Error"
+    end
   end
 
   # Raised when an API object is not found.
@@ -31,6 +35,10 @@ module OSM
     def render_opts
       { :text => "Precondition failed: #{@message}", :status => :precondition_failed, :content_type => "text/plain" }
     end
+
+    def to_s
+      "Precondition failed: #{@message}"
+    end
   end
 
   # Raised when to delete an already-deleted object.
@@ -65,6 +73,10 @@ module OSM
     def render_opts
       { :text => "You need to supply a changeset to be able to make a change", :status => :conflict, :content_type => "text/plain" }
     end
+    
+    def to_s
+       "You need to supply a changeset to be able to make a change"
+    end
   end
 
   # Raised when a diff is uploaded containing many changeset IDs which don't match
@@ -119,6 +131,10 @@ module OSM
         ", server had: " + latest.to_s + " of " + type + " " + id.to_s, 
         :status => :conflict, :content_type => "text/plain" }
     end
+    
+    def to_s
+       "Version mismatch: Provided " + provided.to_s + ", server had: " + latest.to_s + " of " + type + " " + id.to_s
+    end
   end
 
   # raised when a two tags have a duplicate key string in an element.