mod_rest: Improve error handling during format mapping Prevents e.g. a nil, error return going directly into e.g. json encoding, resulting in "null" being returned. Further handling improvements down the line is needed.

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summary: Log a message once per minute
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This module sends `-- MARK --` to the log once per minute. This may be
useful to give a sense of how busy the server is or see that logging and
timers are still working.