ATMO-473: Return conflict messages as 409 errors, and generic errors as 503_SERVICE_UNAVAILABLE, rather than non-standard 500 errors
ATMO-473: Return conflict messages as 409 errors, and generic errors as 503_SERVICE_UNAVAILABLE, rather than non-standard 500 errors

CCI-MOC/GUI-Backend,CCI-MOC/GUI-Backend,CCI-MOC/GUI-Backend,CCI-MOC/GUI-Backend