DEV: show no icon if icon name is missing and log at error level (#31866)

On 1st April 2025, we start showing no icons if the icon name used is a
deprecated one and therefore no longer part of the svg set.

We'll continue showing the messages with the correct icon name to aid
correction of these names.

Console logging will now be done at an error level for such icons.

We retain the behaviour of raising an error for such icons in plugins
from svg_sprite.rb in test environments, but removed this from
icon-library.js as it's harder to test the actual expected behaviour of
returning the original icon now that it's not part of the deprecation
workflow. (sinon.stub doesn't work well here for `isTesting` - the
alternative would be to override the environment.js module with
`proxyquire`) In any case, once we remove the mapping logic, we won't be
raising errors in test environment either for this scenario.

lib/discourse.rb

@@ -117,6 +117,7 @@ module Discourse
 
     class CommandError < RuntimeError
       attr_reader :status, :stdout, :stderr
+
       def initialize(message, status: nil, stdout: nil, stderr: nil)
         super(message)
         @status = status
@@ -305,6 +306,9 @@ module Discourse
   class Deprecation < StandardError
   end
 
+  class MissingIconError < StandardError
+  end
+
   class ScssError < StandardError
   end