sftp: Return error when path starts with a tilde (~)

internal/backend/sftp/config.go

@@ -64,9 +64,15 @@ func ParseConfig(s string) (interface{}, error) {
 	default:
 		return nil, errors.New(`invalid format, does not start with "sftp:"`)
 	}
+
+	p := path.Clean(dir)
+	if strings.HasPrefix(p, "~") {
+		return nil, errors.Fatal("sftp path starts with the tilde (~) character, that fails for most sftp servers.\nUse a relative directory, most servers interpret this as relative to the user's home directory.")
+	}
+
 	return Config{
 		User: user,
 		Host: host,
-		Path: path.Clean(dir),
+		Path: p,
 	}, nil
 }