fix: move notFoundError to notExistError

[fungaren]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:
/kind bug
/kind documentation
/kind enhancement
/kind good-first-issue
/kind feature
/kind question
/kind design
/sig ai
/sig iot
/sig network
/sig storage

What this PR does / why we need it:

If the user set WEBHOOK_HOST environment variable, the webhook certs will be written to filesystem (/tmp/yurt-manager-webhook-certs by default), and the code has a type confusion of notExistError and notFoundError.

This PR also fix a null pointer dereference in updateIfNotExists.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

other Note

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 45.14%. Comparing base (7763e7c) to head (bfe2b82).
Report is 43 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/yurtmanager/webhook/util/writer/certwriter.go	0.00%	4 Missing ⚠️
pkg/yurtmanager/webhook/util/writer/fs.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2213       +/-   ##
===========================================
- Coverage   58.93%   45.14%   -13.80%     
===========================================
  Files         210      402      +192     
  Lines       18968    27754     +8786     
===========================================
+ Hits        11179    12529     +1350     
- Misses       6707    13997     +7290     
- Partials     1082     1228      +146     

Flag	Coverage Δ
unittests	45.14% <0.00%> (-13.80%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[rambohe-ch]

/LGTM

[rambohe-ch]

@fungaren Very appreciated you for posting pull request to fix bug of fs cert management.