`influxd-ctl backup` now validates `-from` node exists

[devanbenz]

ETA: 1.12.3

influxd-ctl backup now validates that the data node specified with -from exists in the cluster before starting the backup. Previously, specifying a non-existent node could lead to confusing errors. Now the command fails immediately with a clear error message: data node "<addr>" does not exist.

InfluxDB Enterprise 1.12.3+