chore: Consolidate error enums and use expect where possible

[adwk67]

Description

Part of stackabletech/issues#883

--- PASS: kuttl (601.98s)
    --- PASS: kuttl/harness (0.00s)
        --- PASS: kuttl/harness/smoke_druid-37.0.0_zookeeper-3.9.5_hadoop-3.5.0_openshift-false (601.97s)
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[siegfriedweber]

In the airflow-operator the expects sit at the call site (pb.add_volumes(…).expect(…)), so the constraint is visible exactly where it has to be honoured: you can see at a glance that nothing may be expected after the user-supplied volumes and volume mounts have been added.

Here most of the adding is delegated to helpers instead: add_tls_volume_and_volume_mounts, add_config_volume_and_volume_mounts, add_log_config_volume_and_volume_mounts, add_log_volume_and_volume_mounts, add_hdfs_cm_volume_and_volume_mounts and RoleResource::update_volumes_and_volume_mounts. From the call site there is no sign that any of them can panic, nor that they must run before the user-supplied additions. The ordering requirement is real, but invisible to anyone editing build_rolegroup_statefulset.

Two ways to address that:

1. Document it on the helpers, e.g.

/// # Panics
///
/// Panics if the volumes or volume mounts cannot be added to the builders. Only call this
/// on builders whose volume names and mount paths are still distinct from the ones added
/// here.

2. Keep the helpers returning Result and move the expects to the call site, as in the airflow-operator.

[adwk67]

I opted for the # Panics docs in b89a8df as otherwise we would need to add back in a fair amount of what has been removed. I'll apply this to the other open PRs as well.