Add 'code', 'message' to ErrorHandler log context.

Passed the 'code' and 'message' from the original error/fatal-error
to the context of ErrorHandler log record(s).
This will help writers of Handlers to save structured information about
errors/fatal-errors. (e.g. Useful for a "DatabaseHandler").
Merge pull request #393 from kerdany/error_handler_patch

Add 'code', 'message' keys to ErrorHandler log record's context array.