Factorise struct.error exception handling with a decorator over decoding

functions. Factorise check of enum based values and string decoding. 
Unify tuple unpacking by always using parenthesis.


git-svn-id: https://svn.erp5.org/repos/neo/branches/prototype3@663 71dcc9de-d417-0410-9af5-da40c76e7ee4