Improve error message for missing LTS binaries (bazelbuild#639)

* Improve error message for missing LTS binaries

Follow-up to bazelbuild#636.

* Remove obsolete import

repositories/gcs.go

@@ -5,7 +5,6 @@ package repositories
 
 import (
 	"encoding/json"
-	"errors"
 	"fmt"
 	"log"
 	"strconv"
@@ -42,7 +41,11 @@ func (gcs *GCSRepo) GetLTSVersions(bazeliskHome string, opts *core.FilterOpts) (
 		return []string{}, err
 	}
 	if len(matches) == 0 {
-		return []string{}, errors.New("there are no LTS releases or candidates")
+		var suffix string
+		if opts.Track > 0 {
+			suffix = fmt.Sprintf(" for track %d", opts.Track)
+		}
+		return []string{}, fmt.Errorf("could not find any LTS Bazel binaries%s", suffix)
 	}
 	return matches, nil
 }