Improve error while retrieving invalid payment methods

Show a simple error after retrieving payment methods no more supported,
suggesting to run a specific rake task to fix the issue and
 without having to delete those payment methods.

core/app/models/spree/payment_method.rb

@@ -11,6 +11,8 @@ module Spree
   # This class is not meant to be instantiated. Please create instances of concrete payment methods.
   #
   class PaymentMethod < Spree::Base
+    class UnsupportedPaymentMethod < StandardError; end
+
     preference :server, :string, default: 'test'
     preference :test_mode, :boolean, default: true
 
@@ -31,6 +33,16 @@ def self.const_missing(name)
       end
     end
 
+    def self.find_sti_class(type_name)
+      super(type_name)
+    rescue ActiveRecord::SubclassNotFound
+      raise UnsupportedPaymentMethod, "Found invalid payment type '#{type_name}'.\n"\
+        "This may happen after switching payment service provider, when payment methods "\
+        "reference old types that are not supported any more.\n"\
+        "If that is the case, consider running 'rake payment_method:deprecate_unsupported_payment_methods' "\
+        "to fix the issue."
+    end
+
     validates :name, :type, presence: true
 
     has_many :payments, class_name: "Spree::Payment", inverse_of: :payment_method

core/spec/models/spree/payment_method_spec.rb

@@ -395,4 +395,22 @@ def gateway_class
       expect(described_class::DISPLAY).to eq([:both, :front_end, :back_end])
     end
   end
+
+  describe "#find_sti_class" do
+    context "with an unexisting type" do
+      context "while retrieving records" do
+        let!(:unsupported_payment_method) { create(:payment_method, type: 'UnsupportedPaymentMethod') }
+        before do
+          unsupported_payment_method.update type: 'UnsupportedPaymentMethod'
+        end
+
+        it "raises an UnsupportedPaymentMethod error" do
+          expect { Spree::PaymentMethod.all.to_json }
+            .to raise_error(
+              Spree::PaymentMethod::UnsupportedPaymentMethod,
+              /Found invalid payment type 'UnsupportedPaymentMethod'/)
+        end
+      end
+    end
+  end
 end