Mention error message in OAuth setup for iD

My usual workflow for trying to fix errors that I don't understand is to paste the error message into a search engine, and I think many other people do the same. Putting the error message here would help me, and other people, find the step missed during setup.

CONFIGURE.md

@@ -86,6 +86,8 @@ potlatch2_key: "8lFmZPsagHV4l3rkAHq0hWY5vV3Ctl3oEFY1aXth"
 
 Follow the same process for registering and configuring iD (`id_key`) and the website/Notes (`oauth_key`), or to save time, simply reuse the same consumer key for each.
 
+**NOTE:** If you forget to set up OAuth, then you will get an error message similar to `uninitialized constant ActionView::CompiledTemplates::ID_KEY`.
+
 ## Troubleshooting
 
 Rails has its own log.  To inspect the log, do this: