* Change http-proxy-middleware logLevel from silent to error

* provide onError handler for httpProxyMiddleware

* Send proper error reponse upon proxy error.

* Adds JSX extension support

* PR changes

* Add testRegex

* Add note about not recommending JSX, link to issue

* Change http-proxy-middleware logLevel from silent to error

* provide onError handler for httpProxyMiddleware

For the 0.4.0 CHANGELOG entry, the npm install command should be:
```
npm install --save-dev --save-exact react-scripts@0.4.0
```

Not:
```
npm install --save-dev --save-exact react-scripts@0.3.1
```

* Clarify proxy instructions in generated app README

* Add backticks to format text/html as code

With the HTTPS env var set 'true', the dev server will serve over HTTPS.

* Disable contentBase in development

* Document #428

env.js is only for variables injected into the app.

Fixes: #253

tagging dev-only code (@remove-on-publish-begin/end) and pre-eject-only code (@remove-on-eject-begin/end) #257 (#257)