Uploads: Check if strategy is enabled before use

Avoid returning bare API tracebacks from pixiv et al when login details
are not configured, and instead raise a generic error.

app/logical/upload_service/preprocessor.rb

@@ -64,6 +64,8 @@ class UploadService
     end
 
     def start!
+      raise NotImplementedError, "No login credentials configured for #{strategy.site_name}." unless strategy.class.enabled?
+
       if Utils.is_downloadable?(source)
         if Post.system_tag_match("source:#{canonical_source}").where.not(id: original_post_id).exists?
           raise ActiveRecord::RecordNotUnique, "A post with source #{canonical_source} already exists"

app/logical/upload_service/utils.rb

@@ -72,6 +72,8 @@ class UploadService
       raise "No file or source URL provided" if upload.source_url.blank?
 
       strategy = Sources::Strategies.find(upload.source_url, upload.referer_url)
+      raise NotImplementedError, "No login credentials configured for #{strategy.site_name}." unless strategy.class.enabled?
+
       file = strategy.download_file!
 
       if strategy.data[:ugoira_frame_data].present?