Treat empty group list is an error response from Keycloak

[smlx]

If you request a list of realm groups from Keycloak via the API with insufficient permissions (required permissions are undocumented) it will sometimes (exact circumstances also undocumented) return an empty groups list with a 200 OK rather than an error response.

Currently, lagoon-opensearch-sync will assume the empty groups list is valid and proceed to remove all tenants from Opensearch Dashboards. Instead, an empty groups list should be treated as an error response, and the error message from lagoon-opensearch-sync should suggest possible solutions.

Related: #149