[#927] neofs-adm: provide better error message if endpoint is missing

Note that we cannot mark `--rpc-endpoint` flag as required because
it can be taken from config.

Before:
```
Error: can't create N3 client: failed to get network magic: Post "": unsupported protocol scheme ""
```

Now:
```
Error: can't create N3 client: missing endpoint
```

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

cmd/neofs-adm/internal/modules/morph/n3client.go

@@ -2,6 +2,7 @@ package morph
 
 import (
 	"context"
+	"errors"
 
 	"github.com/nspcc-dev/neo-go/pkg/rpc/client"
 	"github.com/spf13/viper"
@@ -10,6 +11,9 @@ import (
 func getN3Client(v *viper.Viper) (*client.Client, error) {
 	ctx := context.Background() // FIXME(@fyrchik): timeout context
 	endpoint := v.GetString(endpointFlag)
+	if endpoint == "" {
+		return nil, errors.New("missing endpoint")
+	}
 	c, err := client.New(ctx, endpoint, client.Options{})
 	if err != nil {
 		return nil, err