Error message about non-robust constraint drop is bugged

[sbdchd]

it recommends:

  help: Consider wrapping in a transaction or adding a IF NOT EXISTS clause if the statment supports it.

when it should say

  help: Consider wrapping in a transaction or adding a IF EXISTS clause if the statment supports it.

For dropping a constraint, it should say:

IF EXISTS instead of IF NOT EXISTS

For adding a constraint, it should say:

IF EXISTS (which it does right now)