Tests for PreconditionFailed error messages.

Change to relation.rb to bring element selection in-line with that of way, so to return nil to be caught later, rather than raise an error.
Change to way.rb to clarify which way on diff upload.

app/models/relation.rb

@@ -298,7 +298,7 @@ class Relation < ActiveRecord::Base
         # use reflection to look up the appropriate class
         model = Kernel.const_get(m[0].capitalize)
         # get the element with that ID
-        element = model.find(m[1])
+        element = model.find(:first, :conditions =>["id = ?", m[1]])
 
         # and check that it is OK to use.
         unless element and element.visible? and element.preconditions_ok?