cask/utils: Make more noise when encountering undefined methods

- https://github.com/Homebrew/homebrew-cask/actions/runs/4608585102/jobs/8144571098
  introduced syntax errors for the `mattermost` cask (`autoupdates`
  instead of `auto_updates`), but CI didn't fail so we didn't notice
  until it shipped to users and broke `brew update`.

Library/Homebrew/cask/utils.rb

@@ -104,7 +104,7 @@ module Cask
       message << "during #{section} " if section
       message << "on Cask #{token}."
 
-      opoo "#{message}\n#{error_message_with_suggestions}"
+      ofail "#{message}\n#{error_message_with_suggestions}"
     end
   end
 end

Library/Homebrew/test/cask/dsl_spec.rb

@@ -20,10 +20,10 @@ describe Cask::DSL, :cask do
       end
     end
 
-    it "prints a warning that it has encountered an unexpected method" do
+    it "prints an error that it has encountered an unexpected method" do
       expected = Regexp.compile(<<~EOS.lines.map(&:chomp).join)
         (?m)
-        Warning:
+        Error:
         .*
         Unexpected method 'future_feature' called on Cask unexpected-method-cask\\.
         .*