fix(customrun): return error when customrun is not found during delete

[shubhamrai9122-creator]

Changes

Fixes #3329

Previously, deleteCustomRuns printed a 'not found' message when customRunExists returned an error, but skipped the resource without recording the error into deleteErr. As a result, tkn customrun delete exited 0 even when specified resources were not found.

This commit records lookup errors using multierr.Append to align CustomRun delete behavior with Task and Pipeline delete:

• Missing names still print the message and are skipped.
• Existing names are still deleted and reported.
• The command returns an error if any name was not found.

Submitter Checklist

• Includes tests
• Commit messages follow commit message best practices

Release Notes

Fix tkn customrun delete to return a non-zero exit code when a specified customrun is not found.

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