tkn customrun delete exits 0 and silently succeeds when resource is not found #3329

Description

tkn customrun delete does not fail when one or more given names are missing. It prints a "not found" message and still succeeds. Task and Pipeline delete fail the command in the same situation, in both text mode and -o json/-o yaml.

Steps to Reproduce the Problem

1. Run tkn customrun delete nonexistent -n ns
2. Observe that a "not found" message is printed and the command succeeds

Actual Behavior

• Missing names are printed to stderr and skipped.
• The command still succeeds if no DELETE call fails.
• Names that exist are deleted as usual.

Expected Behavior

• Missing names still print a message and are skipped.
• Names that exist are still deleted and reported.
• The command fails if any name was not found, matching Task and Pipeline delete.

Additional Info

Found in review of #3318.

Task and Pipeline collect lookup errors (taskExists / pipelineExists) and return them after successful deletes. CustomRun prints the lookup error in deleteCustomRuns and continues without recording it.