suppress ServiceUnavailableError for nats

[swelborn]

Description

Suppress another nats error when fetching messages on subscriptions.

ServiceUnavailableError intermittently pops up during jetstream leader elections. My services get stuck in an infinite supervisor loop (see #2632 for more details).

Since the issue is transient it is a bit difficult to test, but one way I have managed to reproduce is to use a jetstream cluster with 3 replicas, connect with a faststream subscriber, then shut down the cluster, and bring them back up one at a time.

I think that since this error is transient, it is in line with a TimeoutError, so it should be fine to suppress.

Fixes # (issue number)

Type of change

Please delete options that are not relevant.

• Documentation (typos, code examples, or any documentation updates)
• Bug fix (a non-breaking change that resolves an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (a fix or feature that would disrupt existing functionality)
• This change requires a documentation update

Checklist

• My code adheres to the style guidelines of this project (just lint shows no errors)
• I have conducted a self-review of my own code
• I have made the necessary changes to the documentation
• My changes do not generate any new warnings
• I have added tests to validate the effectiveness of my fix or the functionality of my new feature
• Both new and existing unit tests pass successfully on my local environment by running just test-coverage
• I have ensured that static analysis tests are passing by running just static-analysis
• I have included code examples to illustrate the modifications