Respond 404 for non-exist theme resources instead of 500

[JohnNiang]

What type of PR is this?

/kind bug
/area core
/milestone 2.18.x

What this PR does / why we need it:

This PR checks readable of theme resources while getting resources to prevent Halo from throwing FileNotFoundException.

Which issue(s) this PR fixes:

Fixes #6338

Special notes for your reviewer:

1. Try to request https://www.halo.run/themes/fake-theme/assets/favicons/favicon-32x32.png
2. See the result

Does this PR introduce a user-facing change?

修复访问不存在的主题资源时出现服务器异常的问题

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 58.34%. Comparing base (bc10336) to head (e64e75e).
Report is 79 commits behind head on main.

Files	Patch %	Lines
.../halo/app/theme/config/ThemeWebFluxConfigurer.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6340      +/-   ##
============================================
+ Coverage     54.51%   58.34%   +3.83%     
- Complexity     3523     3763     +240     
============================================
  Files           646      646              
  Lines         21862    21977     +115     
  Branches       1528     1546      +18     
============================================
+ Hits          11917    12822     +905     
+ Misses         9328     8534     -794     
- Partials        617      621       +4     

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[JohnNiang]

/cherrypick release-2.17

[halo-dev-bot]

@JohnNiang: once the present PR merges, I will cherry-pick it on top of release-2.17 in a new PR and assign it to you.

In response to this:

/cherrypick release-2.17

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[ruibaby]

/lgtm

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ruibaby

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• OWNERS [ruibaby]

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[halo-dev-bot]

@JohnNiang: new pull request created: #6355

In response to this:

/cherrypick release-2.17

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.