explorer should be tolerant to missing crds in management clusters

[enekofb]

this ticket comes in the context of this conversation https://weaveworks.slack.com/archives/C04RDBTQF9B/p1697034724643779

where we want Explorer to start and server as many resources as possible without crashing in case of missing CRDs. this ticket is to verify that scenario and in in case of error fix it:

Scenario

Given Weave Gitops with Explorer
And not enabled gitopssets in management clusters
When I go to Explorer UI

Expected

Then Explorer shows available resources but not gitopssets

Not Expected

Then Explorer has crashed shows available resources but not gitopssets