show error details when fetching children fails (#28)

[adietish]

fixes #28

[adietish]

With this PR the cause is displayed instead of a generic failure text which tells you that the resources could not have been retrieved.

OpenShift, token expired:

OpenShift, fetching resources forbidden (no premissions):

Minikube, connection timed out:

[adietish]

To test this one needs a Red Hat Developer Sandbox with an expired token (ex. create the cluster, try to use it after 24h) and a minikube that's defined in kube config but is not running (ex. start minikube, copy kube config, stop minikube, overwrite kube config with the copy). Then:

1. EXEC: on Red Hat Sandbox: expand "Projects" node in resource tree
   -> child node displays error telling you that the token is expired (see screenshot above)
2. EXEC: on Red Hat Sandbox: re-authenticate so that you have a valid token.
   Then expand "Namespaces" node in resource tree
   -> child node displays error telling you that the request is forbidden (see screenshot above)
3. EXEC. on minikube: list any category
   -> child node displays an error telling you that the request timed out

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
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0 Code Smells

No Coverage information
0.0% Duplication

[sbouchet]

LGTM. getting real cause messages in tree as expected by this PR