apollo client:download-schema can not run more than one time for the same output path

**Assumption:**
`apollo client:download-schema --endpoint=http://localhost:4000 src/generated/schema.json` should always download the schema and place it in the specified file.

**Use case:** Automatically generate SDL file based on Schema JSON, for automatic hinting in VSCode and Intellij in inline GQL queries for React components.

**Not possible because:** If a schema.json file exists, download-schema will for some reason try and validate it before downloading a new schema and exit with !=0 .

**Workaround:** Delete existing schema.json before downloading new one.

Full command I'm working with, triggered on detected schema changes by a watcher:
`apollo client:download-schema --endpoint=http://localhost:4000 src/generated/schema.json | cat src/generated/schema.json | json-to-simple-graphql-schema > src/generated/schema.graphql"`

**Actual outcome:**
After you run `apollo client:download-schema --endpoint=http://localhost:4000 src/generated/schema.json` one time, all subsequent calls error:

```
Saving schema to src/generated/schema.json
→ Name "__schema" must not begin with "__", which is reserved by GraphQL introspection.
```

**How to reproduce the issue:**
Run the command twice

**Versions**
apollo/2.16.1 win32-x64 node-v10.15.1

Start by running client:download-schema twice with the same output path and compare the failure with the first run. Trace the client:download-schema command's existing-output validation and schema download flow. Done means repeated runs overwrite or refresh the specified schema.json successfully without requiring manual deletion.

Written by the indexing model from the issue text.