fix(cognito): ambiguous error message when same trigger is added twice (aws#16917)

when the same trigger is added twice to the Cognito userpool, 
```ts
const fn = lambda.Function.fromFunctionArn(stack, 'Trigger', 'arn:aws:lambda:us-east-1:123456789012:function:CognitoFunction')

const userpool = new cognito.UserPool(stack, 'Userpool', { lambdaTriggers: { customMessage: fn } })

userpool.addTrigger(cognito.UserPoolOperation.CUSTOM_MESSAGE, fn)
```
throws error message:

`Error: A trigger for the operation [object Object] already exists.`

This PR fixes it as:

` Error: A trigger for the operation customMessage already exists.`

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: nom3ad

packages/@aws-cdk/aws-cognito/lib/user-pool.ts

@@ -830,7 +830,7 @@ export class UserPool extends UserPoolBase {
    */
   public addTrigger(operation: UserPoolOperation, fn: lambda.IFunction): void {
     if (operation.operationName in this.triggers) {
-      throw new Error(`A trigger for the operation ${operation} already exists.`);
+      throw new Error(`A trigger for the operation ${operation.operationName} already exists.`);
     }
 
     this.addLambdaPermission(fn, operation.operationName);

packages/@aws-cdk/aws-cognito/test/user-pool.test.ts

@@ -403,7 +403,7 @@ describe('User Pool', () => {
 
     // WHEN
     userpool.addTrigger(UserPoolOperation.CREATE_AUTH_CHALLENGE, fn1);
-    expect(() => userpool.addTrigger(UserPoolOperation.CREATE_AUTH_CHALLENGE, fn2)).toThrow(/already exists/);
+    expect(() => userpool.addTrigger(UserPoolOperation.CREATE_AUTH_CHALLENGE, fn2)).toThrow(/createAuthChallenge already exists/);
   });
 
   test('no username aliases specified', () => {