Auto merge of #18193 - Wilfred:startup_error, r=lnicola
fix: Don't report a startup error when a discover command is configured
Previously, r-a would show an error if both fetch_workspaces_queue and discover_workspace_queue were empty. We're in this state at startup, so users would see an error if they'd configured
discover_workspace_config.
Instead, allow the fetch_workspaces_queue to have zero items if discover_workspace_config is set.
Whilst we're here, prefer "failed to fetch" over "failed to discover", so the error message better reflects what this function is doing.
