Raise database error when table doesn't exist

Since 1944a7e74c, adding a foreign key or
check constraint to a non-existent table results in an unhelpful error:

    NoMethodError: undefined method `scan' for nil:NilClass

This restores the previous behaviour of raising the underlying error
from the database, which describes the actual problem.

activerecord/lib/active_record/connection_adapters/sqlite3/schema_statements.rb

@@ -89,7 +89,7 @@ module ActiveRecord
             WHERE name = #{quote_table_name(table_name)} AND type = 'table'
           SQL
 
-          table_sql.scan(/CONSTRAINT\s+(?<name>\w+)\s+CHECK\s+\((?<expression>(:?[^()]|\(\g<expression>\))+)\)/i).map do |name, expression|
+          table_sql.to_s.scan(/CONSTRAINT\s+(?<name>\w+)\s+CHECK\s+\((?<expression>(:?[^()]|\(\g<expression>\))+)\)/i).map do |name, expression|
             CheckConstraintDefinition.new(table_name, expression, name: name)
           end
         end

activerecord/test/cases/migration/check_constraint_test.rb

@@ -21,7 +21,7 @@ if ActiveRecord::Base.connection.supports_check_constraints?
         end
 
         teardown do
-          @connection.drop_table "trades", if_exists: true
+          @connection.drop_table "trades", if_exists: true rescue nil
         end
 
         def test_check_constraints
@@ -56,6 +56,13 @@ if ActiveRecord::Base.connection.supports_check_constraints?
           end
         end
 
+        def test_add_check_constraint_with_non_existent_table_raises
+          e = assert_raises(ActiveRecord::StatementInvalid) do
+            @connection.add_check_constraint :refunds, "quantity > 0", name: "quantity_check"
+          end
+          assert_match(/refunds/, e.message)
+        end
+
         def test_added_check_constraint_ensures_valid_values
           @connection.add_check_constraint :trades, "quantity > 0", name: "quantity_check"
 

activerecord/test/cases/migration/foreign_key_test.rb

@@ -189,8 +189,8 @@ if ActiveRecord::Base.connection.supports_foreign_keys?
         end
 
         teardown do
-          @connection.drop_table "astronauts", if_exists: true
-          @connection.drop_table "rockets", if_exists: true
+          @connection.drop_table "astronauts", if_exists: true rescue nil
+          @connection.drop_table "rockets", if_exists: true rescue nil
         end
 
         def test_foreign_keys
@@ -308,6 +308,20 @@ if ActiveRecord::Base.connection.supports_foreign_keys?
           assert_equal :nullify, fk.on_update
         end
 
+        def test_add_foreign_key_with_non_existent_from_table_raises
+          e = assert_raises StatementInvalid do
+            @connection.add_foreign_key :missions, :rockets
+          end
+          assert_match(/missions/, e.message)
+        end
+
+        def test_add_foreign_key_with_non_existent_to_table_raises
+          e = assert_raises StatementInvalid do
+            @connection.add_foreign_key :missions, :rockets
+          end
+          assert_match(/missions/, e.message)
+        end
+
         def test_foreign_key_exists
           @connection.add_foreign_key :astronauts, :rockets