feat(assets): throw ValidationError instead of untyped Errors (#33383)

### Issue 

Relates to #32569 

### Description of changes

`ValidationErrors` everywhere

### Describe any new or updated permissions being added

n/a

### Description of how you validated changes

Existing tests. Exemptions granted as this is a refactor of existing code.

### 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: mrgrain

packages/aws-cdk-lib/.eslintrc.js

@@ -16,6 +16,7 @@ baseConfig.rules['import/no-extraneous-dependencies'] = [
 
 // no-throw-default-error
 const enableNoThrowDefaultErrorIn = [
+  'assets',
   'aws-amplify',
   'aws-amplifyuibuilder',
   'aws-apigateway',

packages/aws-cdk-lib/assets/lib/compat.ts

@@ -1,5 +1,5 @@
 import { FollowMode } from './fs/follow-mode';
-import { SymlinkFollowMode } from '../../core';
+import { SymlinkFollowMode, UnscopedValidationError } from '../../core';
 
 export function toSymlinkFollow(follow?: FollowMode): SymlinkFollowMode | undefined {
   if (!follow) {
@@ -12,6 +12,6 @@ export function toSymlinkFollow(follow?: FollowMode): SymlinkFollowMode | undefi
     case FollowMode.BLOCK_EXTERNAL: return SymlinkFollowMode.BLOCK_EXTERNAL;
     case FollowMode.EXTERNAL: return SymlinkFollowMode.EXTERNAL;
     default:
-      throw new Error(`unknown follow mode: ${follow}`);
+      throw new UnscopedValidationError(`unknown follow mode: ${follow}`);
   }
 }