[Feature]: verify barmanObjectName

when referencing an `barmanObjectName ` in `externalCluster` that is used for recovery
and that does not exist
the creation of the cluster stops without error after "creating cluster role" phase

Therefor it would be good to verify the if the barmanObjectName exists

```
...
bootstrap:
#########################
# restore just in case...
#########################

recovery:
source: source
# recoveryTarget:
# targetTime: "2023-06-19 17:15:00.00000+02"

externalClusters:
- name: source
plugin:
name: barman-cloud.cloudnative-pg.io
parameters:
# Recovery Object Store (pull, read-only)
barmanObjectName: minio-store
serverName: xxx
...
```

Trace recovery handling for the externalCluster plugin configuration and the barmanObjectName parameter; no file or test is identified in the issue. Determine where the referenced object is resolved during cluster creation, then verify that a missing object produces an explicit error instead of stopping after the "creating cluster role" phase.

Written by the indexing model from the issue text.