imp: upstream mock app error type included in 7c5384f (#4189)

Co-authored-by: Jim Fasarakis-Hilliard <[email protected]>
Co-authored-by: Carlos Rodriguez <[email protected]>

testing/mock/ibc_module.go

@@ -14,6 +14,13 @@ import (
 	"github.com/cosmos/ibc-go/v7/modules/core/exported"
 )
 
+// applicationCallbackError is a custom error type that will be unique for testing purposes.
+type applicationCallbackError struct{}
+
+func (e applicationCallbackError) Error() string {
+	return "mock application callback failed"
+}
+
 // IBCModule implements the ICS26 callbacks for testing/mock.
 type IBCModule struct {
 	appModule *AppModule

testing/mock/mock.go

@@ -41,6 +41,9 @@ var (
 	MockRecvCanaryCapabilityName    = "mock receive canary capability name"
 	MockAckCanaryCapabilityName     = "mock acknowledgement canary capability name"
 	MockTimeoutCanaryCapabilityName = "mock timeout canary capability name"
+	// MockApplicationCallbackError should be returned when an application callback should fail. It is possible to
+	// test that this error was returned using ErrorIs.
+	MockApplicationCallbackError error = &applicationCallbackError{}
 )
 
 var _ porttypes.IBCModule = IBCModule{}