applicationset: use requeue after if generate app errors out

The `GenerateApplications` can call to external resources like Github
API for instance which might be rate limited or fail. If those requests
somehow fail we should requeue them after some time like (same
reason as https://github.com/argoproj/argo-cd/blob/e98d3b2a877fb7140880dc1450703a99db4398b5/applicationset/controllers/applicationset_controller.go#L154).

For instance, in our environments we were rate limited by Github and the ArgoCD
applicationset controller was logging the following error about every
second or less for every application set using the pull request generator
that we have:
```
time="2024-06-21T14:17:15Z" level=error msg="error generating params" error="error listing repos: error listing pull requests for LedgerHQ/ledger-wordpress-website: GET https://api.github.com/repos/LedgerHQ/xxx/pulls?per_page=100: 403 API rate limit exceeded for installation ID xxx. If you reach out to GitHub Support for help, please include the request ID 44C0:3F7D61:6E73D3:74E1C0:66758B6B and timestamp 2024-06-21 14:17:15 UTC. [rate reset in 8m18s]" generator="&{0xc000d652c0 0x289a100 {0xc00087bdd0}  [] true}"
time="2024-06-21T14:17:15Z" level=error msg="error generating application from params" applicationset=argocd/xxx error="error listing repos: error listing pull requests for LedgerHQ/xxxx: GET https://api.github.com/repos/LedgerHQ/xxx/pulls?per_page=100: 403 API rate limit exceeded for installation ID xxx. If you reach out to GitHub Support for help, please include the request ID xxx and timestamp 2024-06-21 14:17:15 UTC. [rate reset in 8m18s]" generator="{nil nil nil nil nil &PullRequestGenerator{Github:&PullRequestGeneratorGithub{Owner:LedgerHQ,Repo:xxx,API:,TokenRef:nil,AppSecretName:xxxx,Labels:[argocd/preview],},GitLab:nil,Gitea:nil,BitbucketServer:nil,Filters:[]PullRequestGeneratorFilter{},RequeueAfterSeconds:*1800,Template:ApplicationSetTemplate{ApplicationSetTemplateMeta:ApplicationSetTemplateMeta{Name:,Namespace:,Labels:map[string]string{},Annotations:map[string]string{},Finalizers:[],},Spec:ApplicationSpec{Source:nil,Destination:ApplicationDestination{Server:,Namespace:,Name:,},Project:,SyncPolicy:nil,IgnoreDifferences:[]ResourceIgnoreDifferences{},Info:[]Info{},RevisionHistoryLimit:nil,Sources:[]ApplicationSource{},},},Bitbucket:nil,AzureDevOps:nil,} nil nil nil nil}"
```

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@ledger.fr>

applicationset/controllers/applicationset_controller.go

@@ -143,7 +143,7 @@ func (r *ApplicationSetReconciler) Reconcile(ctx context.Context, req ctrl.Reque
 			}, parametersGenerated,
 		)
 		if len(desiredApplications) < 1 {
-			return ctrl.Result{}, generatorsErr
+			return ctrl.Result{RequeueAfter: ReconcileRequeueOnValidationError}, generatorsErr
 		}
 	}