validateInput hook: addValidationError should accept an error code in addition to the message

[remygendron]

Architecturally, the API consumer or client, is responsible for displaying messages in a format aligned with the view:

• Web apps will display a longer message.
• Mobile apps will display a shorter message.
• Programmatic integrations need to make decisions on the error itself.

It is OK to provide a textual message, as this will get displayed in the Admin UI. However, for all the other use cases, an error code is necessary so that the client selects a proper message.

Error codes are also necessary to facilitate I18N of the messages on the client side.

As a work around, I currently prefix my error messages with a code, like:

'listname.fieldname.not.null:The default and admin ui error message.'