Show a better message if a save fails due to a conflict error.

How it works:

  * When the CRUD handler catches a ConflictException, it sets an
    instance variable to indicate the failure.

  * When the update monitor form params are processed by the form
    helper, it adds a data-* variable to the form indicating that the
    record is already known to be stale.

  * The 'poll' of the update monitor JavaScript looks for this flag
    before sending its AJAX query.  If the record is already known to
    be stale, it immediately shows a modified version of the "This
    record has been updated by another user" message.
Stop polling once the record is known to be stale
Merge pull request #472 from hudmol/better-conflict-error

Show a better message if a save fails due to a conflict error.