Fix ambiguous behavior in generate_tokenn method

The `SolidusPaypalBraintree::Gateway#generate_token` method is ambiguous
on success and failure. Both results will return a string. To be more
explicit, this method should raise an error which can be handled by the
app.

app/controllers/solidus_paypal_braintree/client_tokens_controller.rb

@@ -5,7 +5,7 @@ class ClientTokensController < Spree::Api::BaseController
     before_action :load_gateway
 
     def create
-      render json: { client_token: @gateway.generate_token, payment_method_id: @gateway.id }
+      render json: { client_token: generate_token, payment_method_id: @gateway.id }
     end
 
     private
@@ -18,5 +18,12 @@ def load_gateway
         @gateway = ::SolidusPaypalBraintree::Gateway.where(active: true).merge(store_payment_methods_scope).first!
       end
     end
+
+    def generate_token
+      @gateway.generate_token
+    rescue ::SolidusPaypalBraintree::Gateway::TokenGenerationDisabledError => error
+      Rails.logger.error error
+      nil
+    end
   end
 end

app/models/solidus_paypal_braintree/gateway.rb

@@ -4,6 +4,8 @@ module SolidusPaypalBraintree
   class Gateway < ::Spree::PaymentMethod
     include RequestProtection
 
+    class TokenGenerationDisabledError < StandardError; end
+
     # Error message from Braintree that gets returned by a non voidable transaction
     NON_VOIDABLE_STATUS_ERROR_REGEXP = /can only be voided if status is authorized/
 
@@ -213,12 +215,12 @@ def create_profile(payment)
       end
     end
 
+    # @raise [TokenGenerationDisabledError]
+    #   If `preferred_token_generation_enabled` is false
+    #
     # @return [String]
     #   The token that should be used along with the Braintree js-client sdk.
     #
-    #   returns an error message if `preferred_token_generation_enabled` is
-    #   set to false.
-    #
     # @example
     #   <script>
     #     var token = #{Spree::Braintree::Gateway.first!.generate_token}
@@ -233,7 +235,10 @@ def create_profile(payment)
     #     );
     #   </script>
     def generate_token
-      return TOKEN_GENERATION_DISABLED_MESSAGE unless preferred_token_generation_enabled
+      unless preferred_token_generation_enabled
+        raise TokenGenerationDisabledError, TOKEN_GENERATION_DISABLED_MESSAGE
+      end
+
       braintree.client_token.generate
     end
 

spec/models/solidus_paypal_braintree/gateway_spec.rb

@@ -617,7 +617,7 @@
         gateway
       end
 
-      it { is_expected.to match(/Token generation is disabled/) }
+      it { expect { subject }.to raise_error SolidusPaypalBraintree::Gateway::TokenGenerationDisabledError }
     end
   end
 end