Adds TYPICAL_ERRORS' array and adds back safe navigation operator &…

….` to `results = response&.`:

- Prevents:  'NameError:
       uninitialized constant ArticleImporter::TYPICAL_ERRORS' when `ApiErrorHandling` sets the error level
- Prevents: 'NoMethodError:
       undefined method `dig' for nil:NilClass
             results = response.dig('query', 'pages')
                               ^^^^' when response is nil ( for example when methods that call `import_articles_by_title` are mocked, the api_post response becomes `nil`)

lib/importers/article_importer.rb

@@ -35,7 +35,7 @@ def import_articles_by_title(titles)
         titles: some_article_titles.join('|')
       }
       response = api_post(@wiki.api_url, params)
-      results = response.dig('query', 'pages')
+      results = response&.dig('query', 'pages')
       next if results.blank?
       import_articles_from_title_query(results)
     end
@@ -102,4 +102,8 @@ def too_many_requests?(e)
     return false unless e.instance_of?(MediawikiApi::HttpError)
     e.status == 429
   end
+
+  TYPICAL_ERRORS = [Net::ReadTimeout,
+                    MediawikiApi::HttpError,
+                    MediawikiApi::ApiError].freeze
 end