PBM status is unhelpful/incorrect when missing configurations

When deploying a sharded cluster of mongodb using terraform, integrations happen very quickly.

During this period the s3 integration is made (but its configurations are not set) , when the PBM status is retrieved inside the charm with `charmed-mongodb.pbm status -o json`, it results in `{"Error":"get status of pitr: unable check PITR config status: get config: missed config"}`, this error results in the charm showing a status of `PBM error`

when in fact it should result in something more informative. Perhaps a `BlockedStatus("Missing configurations")`

Start by reproducing the sharded MongoDB deployment with Terraform and inspect the charm's handling of `charmed-mongodb.pbm status -o json` when the S3 integration exists without configuration. Trace how the `missed config` error becomes `PBM error`; done means missing PBM configuration produces a more informative blocked status, such as `Missing configurations`.

Written by the indexing model from the issue text.