CSI: capability check ListVolumes at RPC for nicer error messages

The plugin stub object does not include fine-grained capability checks, which
means `nomad volume status -verbose` will return ugly and verbose error
"Unimplemented" messages from the plugin if it does not support the CSI
`ListVolumes` RPC. Return a nicer error message from our RPC handler instead.

nomad/csi_endpoint.go

@@ -1027,6 +1027,9 @@ func (v *CSIVolume) ListExternal(args *structs.CSIVolumeExternalListRequest, rep
 	if plugin == nil {
 		return fmt.Errorf("no such plugin")
 	}
+	if !plugin.HasControllerCapability(structs.CSIControllerSupportsListVolumes) {
+		return fmt.Errorf("unimplemented for this plugin")
+	}
 
 	method := "ClientCSI.ControllerListVolumes"
 	cReq := &cstructs.ClientCSIControllerListVolumesRequest{

nomad/csi_endpoint_test.go

@@ -964,6 +964,7 @@ func TestCSIVolumeEndpoint_ListExternal(t *testing.T) {
 			Healthy:  true,
 			ControllerInfo: &structs.CSIControllerInfo{
 				SupportsAttachDetach: true,
+				SupportsListVolumes:  true,
 			},
 			RequiresControllerPlugin: true,
 		},