[FLINK-30417] [Connectors/ RabbitMQ] add "RabbitMQ" to rabbit error while creating the channel

[smulikHakipod]

Jira:

https://issues.apache.org/jira/browse/FLINK-30417

What is the purpose of the change

To better describe the error of RabbitMQ connection failures.

Brief change log

• Better error indication when we fail to connect to RabbitMQ

Verifying this change

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

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

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): ( no)
• The serializers: ( no )
• The runtime per-record code paths (performance sensitive): ( no )
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: ( no )
• The S3 file system connector: ( no )

Documentation

• Does this pull request introduce a new feature? ( no)
• If yes, how is the feature documented? (not applicable )

[flinkbot]

CI report:

• c3c666f UNKNOWN

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[MartijnVisser]

@smulikHakipod Please redirect this PR to https://github.com/apache/flink-connector-rabbitmq as connectors are being externalized

[smulikHakipod]

apache/flink-connector-rabbitmq#4