fix(core): Tag must have a value error is impossible to attribute to a specific tag (#35091)

### Issue # (if applicable)

Not applicable

### Reason for this change



Make it clearer as to what tag has invalid values - currently the process to identify said tag is not as clear as it could be.

### Description of changes

Improve error message from `Tag must have a value` to `Tag '${key}' must have a value`

### Describe any new or updated permissions being added


Not applicable

### Description of how you validated changes



Test added

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

Author: ryan-pol

packages/aws-cdk-lib/core/lib/tag-aspect.ts

@@ -121,7 +121,7 @@ export class Tag extends TagBase {
   constructor(key: string, value: string, props: TagProps = {}) {
     super(key, props);
     if (value === undefined) {
-      throw new UnscopedValidationError('Tag must have a value');
+      throw new UnscopedValidationError(`Tag '${key}' must have a value`);
     }
     this.value = value;
   }

packages/aws-cdk-lib/core/test/tag-aspect.test.ts

@@ -13,6 +13,7 @@ import {
   ITaggable,
   ITaggableV2,
   AspectPriority,
+  UnscopedValidationError,
 } from '../lib';
 import { synthesize } from '../lib/private/synthesis';
 
@@ -331,4 +332,14 @@ describe('tag aspect', () => {
       }).toThrow();
     });
   });
+
+  test('if tag value is undefined, it raises with appropriate content', () => {
+    expect(() => {
+      new Tag('test-key', undefined as any);
+    }).toThrow(UnscopedValidationError);
+
+    expect(() => {
+      new Tag('test-key', undefined as any);
+    }).toThrow("Tag 'test-key' must have a value");
+  });
 });