avoid nil pointer exception when failing to get listener

instead of an opaque NPE on the next line, panic with explicit error
telling you what went wrong (in my case it was too many open files)

cluster_internal_test.go

@@ -30,6 +30,7 @@ import (
 	"github.com/pilosa/pilosa/v2/testhook"
 	"github.com/pilosa/pilosa/v2/topology"
 	. "github.com/pilosa/pilosa/v2/vprint" // nolint:staticcheck
+	"github.com/pkg/errors"
 )
 
 // GlobalPortMap avoids many races and port conflicts when setting
@@ -53,7 +54,10 @@ func NewGlobalPortMapper(n int) (pm *GlobalPortMapper) {
 		availPorts: make(map[int]net.Listener),
 	}
 	for i := 0; i < n; i++ {
-		lsn, _ := net.Listen("tcp", ":0")
+		lsn, err := net.Listen("tcp", ":0")
+		if err != nil {
+			panic(errors.Wrap(err, "trying to listen on ephemeral port"))
+		}
 		r := lsn.Addr()
 		port := r.(*net.TCPAddr).Port
 		pm.availPorts[port] = lsn