Attempt model annotation only if table exists

Say a developer begins constructing `class Foo < ActiveRecord::Base` and `class Bar < ActiveRecord::Base`. When she creates a migration to create a table for `Foo`, does `rake db:migrate`, she'll get an error from Annotate via the ActiveRecord adapter explaining that the `bars` table doesn't exist. Regardless of whether this is a good development practice, the error message is an annoyance.