SnapsPanic on empty ChainName

after https://github.com/erigontech/erigon/issues/13017

- add panic to:
```
if cfg.ChainName == "" {
log.Debug("[dbg] NewCaplinSnapshots created with empty ChainName", "stack", dbg.Stack())
}
```
or somehow avoid usage of ChainName by `KnownCfg` method

- add panic to:
```
func Seedable(networkName string, info snaptype.FileInfo) bool {
if networkName == "" {
return false
}
```

- same about
```
if cfg.ChainName == "" {
log.Debug("[dbg] newRoSnapshots created with empty ChainName", "stack", dbg.Stack())
}

```