Improve cross-modification error message for undefined gitlab_schema

This can cause confusion for developers adding new tables at present
because the error message doesn't make it clear that the problem (and
hence solution) is just about the table being missing from
`spec/support/database/gitlab_schemas.yml`.

spec/support/database/prevent_cross_database_modification.rb

@@ -102,10 +102,15 @@ module Database
       schemas = Database::GitlabSchema.table_schemas(all_tables)
 
       if schemas.many?
-        raise Database::PreventCrossDatabaseModification::CrossDatabaseModificationAcrossUnsupportedTablesError,
-          "Cross-database data modification of '#{schemas.to_a.join(", ")}' were detected within " \
+        message = "Cross-database data modification of '#{schemas.to_a.join(", ")}' were detected within " \
           "a transaction modifying the '#{all_tables.to_a.join(", ")}' tables." \
           "Please refer to https://docs.gitlab.com/ee/development/database/multiple_databases.html#removing-cross-database-transactions for details on how to resolve this exception."
+
+        if schemas.any? { |s| s.to_s.start_with?("undefined") }
+          message += " The gitlab_schema was undefined for one or more of the tables in this transaction. Any new tables must be added to spec/support/database/gitlab_schemas.yml ."
+        end
+
+        raise Database::PreventCrossDatabaseModification::CrossDatabaseModificationAcrossUnsupportedTablesError, message
       end
     end
   end

spec/support_specs/database/prevent_cross_database_modification_spec.rb

@@ -68,6 +68,17 @@ RSpec.describe 'Database::PreventCrossDatabaseModification' do
         end
       end.to raise_error /Cross-database data modification/
     end
+
+    it 'raises an error when an undefined gitlab_schema table is modified with another table' do
+      expect do
+        with_cross_database_modification_prevented do
+          Project.transaction do
+            project.touch
+            project.connection.execute('UPDATE foo_bars_undefined_table SET a=1 WHERE id = -1')
+          end
+        end
+      end.to raise_error /Cross-database data modification.*The gitlab_schema was undefined/
+    end
   end
 
   context 'when running tests with prevent_cross_database_modification', :prevent_cross_database_modification do