Make directory-not-empty error more self-explanatory

When I encountered the error, I had to go to the source to understand
the problem.  Expanded the message.

Author: justinsb

pkg/plugins/golang/v3/init.go

@@ -17,7 +17,6 @@ limitations under the License.
 package v3
 
 import (
-	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -188,7 +187,7 @@ func checkDir() error {
 				return err
 			}
 			if info.Name() != "go.mod" && !strings.HasPrefix(info.Name(), ".") {
-				return errors.New("only the go.mod and files with the prefix \"(.)\" are allowed before the init")
+				return fmt.Errorf("target directory is not empty: only go.mod and files with the prefix \".\" can exist before init (found %q)", info.Name())
 			}
 			return nil
 		})