application controller: clean up exception handling.

* Simplify code.
* Show backtraces for all users, not just builders.
* Show backtraces only for unexpected server errors (status 5xx), not
  for normal client errors (status 4xx).
* Log expected errors at info level (reduce noise in production logs).

app/views/static/error.js.erb

@@ -1,2 +1,2 @@
-var message = <%= raw @error_message.try(:to_json) || @exception.message.to_json %>;
+var message = <%= raw @message.to_json %>;
 Danbooru.Utility.error(message);