Fix LogCursor error message handler

Serializing this to JSON would dump some odd text with "iseq".

lib/gitlab/geo/project_log_helpers.rb

@@ -8,7 +8,7 @@ module Gitlab
 
       def log_error(message, error)
         data = base_log_data(message)
-        data[:error] = error
+        data[:error] = error.to_s
         Gitlab::Geo::Logger.error(data)
       end