lily-add|update-index: add option to log full stack trace when indexerconf contains error

[bruno]

See subject, this would make it more easier to debug situations like this:
​http://groups.google.com/group/lily-discuss/browse_thread/thread/beabe105698bc732

Possibilities include:

• print this when verbose option is specified
• log the errors using a normal logger instance (e.g. at info level), and allow to specify logging options like for the kauri runtime. This would helpful in other situations too.