metadata-init silently does nothing when --group-name parameter it is omitted

[vsbogd]

Expected behavior: snet-cli fails when --group-name is omitted in metadata-init command.
Actual behavior: snet-cli creates service_metadata.json without payment address:

$ snet service metadata-init service_spec example-executable-service --endpoints http://54.203.198.53:7018 --fixed-price 0 --service-type process --encoding json
$ cat service_metadata.json 
{
    "version": 1,
    "display_name": "example-executable-service",
    "encoding": "json",
    "service_type": "process",
    "model_ipfs_hash": "QmfBoP9CJX59nTZzSg6mJHztqMBMZ8BVRTNZYyV7JhP7tv",
    "mpe_address": "0x5E3b04DbA48B775fCAE65D738b7a75589A42FD3a",
    "groups": [],
    "assets": {}
}

[vsbogd]

related to #324