Use correct error for non-existing email track

This is changing a `NotImplementedError` to an `ArgumentError` to match
the actual error.

app/services/namespaces/in_product_marketing_emails_service.rb

@@ -27,7 +27,7 @@ module Namespaces
     end
 
     def execute
-      raise NotImplementedError, "Track #{track} not defined" unless TRACKS.key?(track)
+      raise ArgumentError, "Track #{track} not defined" unless TRACKS.key?(track)
 
       groups_for_track.each_batch do |groups|
         groups.each do |group|

spec/services/namespaces/in_product_marketing_emails_service_spec.rb

@@ -218,7 +218,7 @@ RSpec.describe Namespaces::InProductMarketingEmailsService, '#execute' do
       stub_const("#{described_class}::TRACKS", { bar: :git_write })
     end
 
-    it { expect { subject }.to raise_error(NotImplementedError, 'Track foo not defined') }
+    it { expect { subject }.to raise_error(ArgumentError, 'Track foo not defined') }
   end
 
   context 'when group is a sub-group' do