[#718] Use error codes instead of relying on english error messages in datastore
[#718] Move pg error codes in a separate dictionary
[#718] Try to avoid PL/pgSQL since we cannot guarantee that is is activated
Merge pull request #719 from okfn/718-datastore-depends-on-localisation-for-2.1

[#718] Use error codes instead of relying on english error messages in d...