bug: false positive error at argument type

The following `catalog.arb` file produces 2 false positive errors:
```
{
"@@locale": "en",
"examplemsg": "Your booking for {var0, plural, one {# guest} other {# guests}} on {var1, date, short} is confirmed.",
"@examplemsg": {
"placeholders": {
"var0": {
"type": "num",
"description": "cardinal plural",
"example": "2"
},
"var1": {
"type": "DateTime",
"description": "time",
"isCustomDateFormat": true
}
}
}
}

```

errors:
```
[{
"resource": "/Users/romshark/Work/tst/catalog.arb",
"owner": "arb",
"code": "missing_other_in_icu",
"severity": 8,
"message": "The ICU message format requires a 'other' argument.",
"startLineNumber": 3,
"startColumn": 85,
"endLineNumber": 3,
"endColumn": 102
},{
"resource": "/Users/romshark/Work/tst/catalog.arb",
"owner": "arb",
"code": "unknown_icu_message_type",
"severity": 8,
"message": "Unknown ICU messagetype \"date\"",
"startLineNumber": 3,
"startColumn": 91,
"endLineNumber": 3,
"endColumn": 95
}]
```