Check whether the namespaces specified in namespace filter exist.

[blackpiglet]

Check whether the namespaces specified in the backup.Spec.IncludeNamespaces exist during backup resource collection.
If not, log an error message to mark the backup as PartiallyFailed.

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Please add a summary of your change

Create a backup with a namespace filter, and specify a non-existing namespace in the filter.
The backup fails with a validation error.

velero backup create --include-namespaces=kube-public,invalid invalid-tst
Backup request "invalid-tst" submitted successfully.
Run `velero backup describe invalid-tst` or `velero backup logs invalid-tst` for more details.

k -n velero get backup invalid-tst  -o yaml
apiVersion: velero.io/v1
kind: Backup
metadata:
  annotations:
    velero.io/resource-timeout: 10m0s
    velero.io/source-cluster-k8s-gitversion: v1.29.4-gke.1043002
    velero.io/source-cluster-k8s-major-version: "1"
    velero.io/source-cluster-k8s-minor-version: "29"
  creationTimestamp: "2024-07-02T03:01:38Z"
  generation: 2
  labels:
    velero.io/storage-location: default
  name: invalid-tst
  namespace: velero
  resourceVersion: "100954432"
  uid: ee6eb635-1fe2-4d3f-8278-050c813fe042
spec:
  csiSnapshotTimeout: 10m0s
  defaultVolumesToFsBackup: false
  hooks: {}
  includedNamespaces:
  - kube-public
  - invalid
  itemOperationTimeout: 4h0m0s
  metadata: {}
  snapshotMoveData: false
  storageLocation: default
  ttl: 720h0m0s
  volumeSnapshotLocations:
  - default
status:
  expiration: "2024-08-01T03:01:38Z"
  formatVersion: 1.1.0
  phase: FailedValidation
  validationErrors:
  - 'Invalid included/excluded namespace lists: Namespace "invalid" not found'
  version: 1

This is not an ideal behavior. The preferred behavior should be making the backup as PartiallyFailed and continues the backup process.
#7928 (comment)

Does your change fix a particular issue?

Fixes #7928

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• Updated the corresponding documentation in site/content/docs/main.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.81%. Comparing base (28d64c2) to head (cf5dfdf).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7965      +/-   ##
==========================================
- Coverage   58.83%   58.81%   -0.02%     
==========================================
  Files         345      345              
  Lines       28795    28796       +1     
==========================================
- Hits        16941    16936       -5     
- Misses      10425    10430       +5     
- Partials     1429     1430       +1     

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