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`.

activerecord/CHANGELOG.md

@@ -1042,4 +1042,10 @@
 
     *Slava Markevich*
 
+*   Raise `ActiveRecord::RecordNotDestroyed` when a replaced child marked with `dependent: destroy` fails to be destroyed.
+
+    Fix #12812
+
+    *Brian Thomas Storti*
+
 Please check [4-0-stable](https://github.com/rails/rails/blob/4-0-stable/activerecord/CHANGELOG.md) for previous changes.

activerecord/lib/active_record/associations/has_many_association.rb

@@ -108,7 +108,7 @@ def inverse_updates_counter_cache?(reflection = reflection)
         # Deletes the records according to the <tt>:dependent</tt> option.
         def delete_records(records, method)
           if method == :destroy
-            records.each { |r| r.destroy }
+            records.each(&:destroy!)
             update_counter(-records.length) unless inverse_updates_counter_cache?
           else
             if records == :all

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -1770,4 +1770,15 @@ def test_collection_association_with_private_kernel_method
     assert_equal [bulb1], car.bulbs
     assert_equal [bulb1, bulb2], car.all_bulbs.sort_by(&:id)
   end
+
+  test "raises RecordNotDestroyed when replaced child can't be destroyed" do
+    car = Car.create!
+    original_child = FailedBulb.create!(car: car)
+
+    assert_raise(ActiveRecord::RecordNotDestroyed) do
+      car.failed_bulbs = [FailedBulb.create!]
+    end
+
+    assert_equal [original_child], car.reload.failed_bulbs
+  end
 end

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

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