[transaction] Avoid a panic when using transaction

[Gilthoniel]

Motivation

When aborting or committing a transaction, it can happen that an error returned from the client triggers a panic when attempting to cast it to a Pulsar error.

Modifications

A proper check is performed when casting the error.

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

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

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

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): no
• The public API: no
• The schema: no
• The default values of configurations: no
• The wire protocol: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented?
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation