Respond not found if no theme template found

[JohnNiang]

What type of PR is this?

/kind improvement
/area core
/milestone 2.19.0

What this PR does / why we need it:

This PR refactors "Template Not Found Exception" into "NotFoundException" to prevent too many exception stacktraces in logs file.

Which issue(s) this PR fixes:

Fixes #6501

Special notes for your reviewer:

1. Activate default theme
2. Request http://localhost:8090/categories
3. See the result

Does this PR introduce a user-facing change?

优化当主题模板找不到的异常提示

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

Attention: Patch coverage is 73.52941% with 9 lines in your changes missing coverage. Please review.

Project coverage is 58.37%. Comparing base (6cd8dc8) to head (e6a5a01).
Report is 14 commits behind head on main.

Files	Patch %	Lines
...main/java/run/halo/app/theme/HaloViewResolver.java	40.00%	8 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6511      +/-   ##
============================================
+ Coverage     58.18%   58.37%   +0.18%     
- Complexity     3774     3790      +16     
============================================
  Files           651      652       +1     
  Lines         22125    22161      +36     
  Branches       1538     1539       +1     
============================================
+ Hits          12873    12936      +63     
+ Misses         8641     8613      -28     
- Partials        611      612       +1     

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[guqing]

/lgtm

[f2c-ci-robot]

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