Fixed messages of exceptions raised by configuration datatypes

Ignore-this: c36250fac07cc7b597f138176ff8cd96

darcs-hash:20090319145300-1e3d8-867cd3152264bc9a4d81f616d3e896bb8d3d5976
Added check for enabled flag of account when setting default account

Ignore-this: 1c7860fd7ea490b0d09030c730a6bd04

darcs-hash:20090319145854-1e3d8-ba4e139ddab6743b32092bd0e86671fa74034004