[schematics] more details about the missing fields

Error: Schematic "init" is missing fields.
what exactly the missing fields?

```
//collection.json
{
"$schema": "../../../node_modules/@angular-devkit/schematics/collection-schema.json",
"schematics": {
"init": {
"description": "creates a new builder",
"factory": "./index#init",
"schema": "./init.schema.json"
}
}
}
```
modify:
after houres of inspecting, the issue was from another schematic ( i.e: using externalSchematic() )
so, it is better to explain whitch schematic that this issue belongs to

先從解析 `collection.json` 中的 `init` 項目時涉及的 Schematics 錯誤路徑開始，然後重現 `externalSchematic()` 呼叫另一個 schematic 的情況。追蹤 `init.schema.json` 和外部 schematic 的識別方式；當錯誤訊息指出缺少欄位所屬的 schematic 時，即表示完成。

由索引模型根據 Issue 內容生成。