Improve the error message for getIfFields
Improve the error message for getIfFields

Patch by: Sverre Rabbelier
Reviewed by: to-be-reviewed

--HG--
extra : convert_revision : svn%3A32761e7d-7263-4528-b7be-7235b26367ec/trunk%40856

rhyolight/nupic.son,rhyolight/nupic.son,rhyolight/nupic.son