fix(kuma-cp): replace err with log when TargetRef can't be resolved

[lobkovilya]

Checklist prior to review

If we return an error when unable to resolve TargetRef (i.e. typo in MeshHTTPRoute name) then:

• Unable to build DPP config
• Inspect API returns error

Current PR replaces err with a log. Ideally we need a mechanism like #5870 to process such kind of errors.

• Link to relevant issue as well as docs and UI issues --
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as a image registry) and it will work on Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS --
• Tests (Unit test, E2E tests, manual test on universal and k8s) --
• Do you need to update UPGRADE.md? --
• Does it need to be backported according to the backporting policy? --
• Do you need to explicitly set a > Changelog: entry here or add a ci/ label to run fewer/more tests?