fix: validate path

See https://git.vvvvvvaria.org/decentral1se/distribusi-go/issues/13.

distribusi.go

@@ -267,6 +267,7 @@ Example:
 			return nil
 		},
 		Action: func(c *cli.Context) error {
+
 			if c.Bool("serve") && c.String("publish") != "" {
 				logrus.Fatal("woops, can't publish & serve at the same time?")
 			}
@@ -276,6 +277,10 @@ Example:
 				logrus.Fatal(err)
 			}
 
+			if _, err := os.Stat(root); os.IsNotExist(err) {
+				logrus.Fatalf("%s does not exist?", root)
+			}
+
 			logrus.Debugf("selecting %s as distribusi root", root)
 
 			var ignore []string