Update the log-levels of some messages

[skx]

This pull-request rejigs a couple of the logged messages:

• Changes a couple of Warning-level messages to be Debug-level.
  • They're not really warnings per se, and they can be hidden by default.
• Upgrades a few Warning-level messages to be Error-level.
  • Failures to send emails, etc, are errors not warnings.
  • Basically every Warn-level message which contains the text message "error" should be an error rather than a warning.

This pull-request updates #143, although it does not resolve it.