[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

Start with the schematics error path involved when resolving the `init` entry in `collection.json`, then reproduce the case where `externalSchematic()` invokes another schematic. Trace how `init.schema.json` and the external schematic are identified; done means the error names the schematic that owns the missing fields.

Written by the indexing model from the issue text.