Better error handling for some of wwctl commands

Better error handling, also prints a stack trace for `wwctl` if
`--debug` flag is used.

cmd/wwctl/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"fmt"
 	"os"
 
 	"github.com/hpcng/warewulf/internal/app/wwctl"
@@ -11,6 +12,10 @@ func main() {
 
 	err := root.Execute()
 	if err != nil {
+		fmt.Fprintf(os.Stderr, "%s: %s\n", os.Args[0], err)
+		if wwctl.DebugFlag {
+			fmt.Printf("\nSTACK TRACE: %+v\n", err)
+		}
 		os.Exit(255)
 	}
 }