Consider adding callback for load or reload errors

[divega]

Currently reload happens in a separate callback and if there are issues in configuration (required files don't exist, or have parsing errors) there is no good way to find out.

This issue is about creating a mechanism that customers can use to execute some action (e.g. log the error message) if such error happens during reload. Potentially this could be used to handle issues on the first load, but there are problems with that because errors during reload are always non-fatal while issues during load are currently always fatal.

[HaoK]

36ec5b0