[fix][broker] Log exception in PulsarMetadataEventSynchronizer failure path

[SongOf]

Motivation

In PulsarMetadataEventSynchronizer, the message listener has three failure paths (single-listener
exceptionally, multi-listener waitForAll exceptionally, and the surrounding catch block).
All three logged the warning "Failed to synchronize for" without attaching the caught exception,
so when metadata-event synchronization failed there was no stack trace or error cause in the broker
log, making the failure impossible to diagnose. The message text was also truncated/incomplete.

Modifications

• Attach the caught exception to the slog warn call via .exception(ex) / .exception(e) in all
  three failure paths of the metadata-event message listener in PulsarMetadataEventSynchronizer.
• Fix the incomplete log message: "Failed to synchronize for" → "Failed to synchronize metadata event".

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment