Fix errors during configuration update when reading obsolete options.

Thix fix makes old confiuration options, for which there is no Option instance in the code anymore, accessible.

Fixes PICARD-642.
Fixed syntax error
Refactored ConfigSection.value to include proper exception handling.
Merge pull request #388 from phw/PICARD-642-config-upgrade-error

Fix errors during configuration update when reading obsolete options.