FIX: logspam due to 404s on CSS files

We had a missing formats: string on our render partial that caused logs to
spam when CSS files got 404s.

Due to magic discourse_public_exceptions.rb was actually returning the
correct 404 cause it switched format when rendering the error.

spec/support/fake_logger.rb

@@ -1,10 +1,11 @@
 class FakeLogger
-  attr_reader :warnings, :errors, :infos
+  attr_reader :warnings, :errors, :infos, :fatals
 
   def initialize
     @warnings = []
     @errors = []
     @infos = []
+    @fatals = []
   end
 
   def info(message = nil)
@@ -18,4 +19,11 @@ class FakeLogger
   def error(message)
     @errors << message
   end
+
+  def fatal(message)
+    @fatals << message
+  end
+
+  def formatter
+  end
 end