[openstreetmap/openstreetmap-website] Mention error message in OAuth setup for iD (#1830)

[mmd]

Mapping a non existing ID_KEY to a blank value would show the following error popup when starting iD:

![grafik](https://user-images.githubusercontent.com/5842757/39259636-37e207ea-48b7-11e8-93a5-5dd25864820e.png)


```ruby
diff --git a/app/views/site/_id.html.erb b/app/views/site/_id.html.erb
index 867b148..57a8967 100644
--- a/app/views/site/_id.html.erb
+++ b/app/views/site/_id.html.erb
@@ -1,7 +1,7 @@
 <%= javascript_include_tag "edit/id" %>
 
 <div id="map">
-  <% data = { :key => ID_KEY } -%>
+  <% data = { :key => defined?(ID_KEY) ? ID_KEY : "" } -%>
   <% data[:lat] = @lat if @lat -%>
   <% data[:lon] = @lon if @lon -%>
   <% data[:gpx] = trace_data_url(params[:gpx], :format => :xml) if params[:gpx] -%>


```

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