Panic when Invalid Struct is Registered (#361)

Without this, an invalid struct can be partially registered in tests or
in workers, which can lead to weird failure when activities are missing
but unknown as to why they are missing.

internal/internal_worker.go

@@ -531,7 +531,10 @@ func (r *registry) RegisterActivityWithOptions(
 	// Validate that it is a function
 	fnType := reflect.TypeOf(af)
 	if fnType.Kind() == reflect.Ptr && fnType.Elem().Kind() == reflect.Struct {
-		_ = r.registerActivityStructWithOptions(af, options)
+		registerErr := r.registerActivityStructWithOptions(af, options)
+		if registerErr != nil {
+			panic(registerErr)
+		}
 		return
 	}
 	if err := validateFnFormat(fnType, false); err != nil {