Raise `RecordNotDestroyed` when children can't be replaced

Fixes #12812
Raise `ActiveRecord::RecordNotDestroyed` when a child marked with
`dependent: destroy` can't be destroyed.

The following code:
```ruby
class Post < ActiveRecord::Base
  has_many :comments, dependent: :destroy
end

class Comment < ActiveRecord::Base
  before_destroy do
    return false
  end
end

post = Post.create!(comments: [Comment.create!])
post.comments = [Comment.create!]
````

would result in a `post` with two `comments`.
With this commit, the same code would raise a `RecordNotDestroyed`
exception, keeping the `post` with the same `comment`.

2 files changed, 7 insertions, 0 deletions

diff --git a/activerecord/test/models/bulb.rb b/activerecord/test/models/bulb.rb
index 4361188e21..831a0d5387 100644
--- a/activerecord/test/models/bulb.rb
+++ b/activerecord/test/models/bulb.rb
@@ -43,3 +43,9 @@ class FunkyBulb < Bulb
     raise "before_destroy was called"
   end
 end
+
+class FailedBulb < Bulb
+  before_destroy do
+    false
+  end
+end
diff --git a/activerecord/test/models/car.rb b/activerecord/test/models/car.rb
index 8f3b70a7c6..c4a15a79e2 100644
--- a/activerecord/test/models/car.rb
+++ b/activerecord/test/models/car.rb
@@ -2,6 +2,7 @@ class Car < ActiveRecord::Base
   has_many :bulbs
   has_many :all_bulbs, -> { unscope where: :name }, class_name: "Bulb"
   has_many :funky_bulbs, class_name: 'FunkyBulb', dependent: :destroy
+  has_many :failed_bulbs, class_name: 'FailedBulb', dependent: :destroy
   has_many :foo_bulbs, -> { where(:name => 'foo') }, :class_name => "Bulb"
 
   has_one :bulb