Raise ArgumentError when no attribute is given to AMo::EachValidator

ArgumentError is better suited than RuntimeError for this.

activemodel/lib/active_model/validator.rb

@@ -135,7 +135,7 @@ class EachValidator < Validator #:nodoc:
     # and instead be made available through the +attributes+ reader.
     def initialize(options)
       @attributes = Array(options.delete(:attributes))
-      raise ":attributes cannot be blank" if @attributes.empty?
+      raise ArgumentError, ":attributes cannot be blank" if @attributes.empty?
       super
       check_validity!
     end

activemodel/test/cases/validations/with_validation_test.rb

@@ -151,7 +151,7 @@ def check_validity!
   end
 
   test "each validator expects attributes to be given" do
-    assert_raise RuntimeError do
+    assert_raise ArgumentError do
       Topic.validates_with(ValidatorPerEachAttribute)
     end
   end