Return 404 instead of 403 for invalid routes

For invalid routes, chalice should return 404 instead of 403

```py
from chalice import Chalice

app = Chalice(app_name='helloworld')

app.debug = True

@app.route('/')
def index():
return {'hello': 'world'}
```

```sh
http :8000
HTTP/1.1 200 OK
Content-Length: 18
Content-Type: application/json
Date: Wed, 31 Oct 2018 09:40:45 GMT
Server: BaseHTTP/0.6 Python/3.6.6

{
"hello": "world"
}

http :8000/hello
HTTP/1.1 403 Forbidden
Content-Length: 43
Content-Type: application/json
Date: Wed, 31 Oct 2018 09:38:10 GMT
Server: BaseHTTP/0.6 Python/3.6.6
x-amzn-ErrorType: UnauthorizedException
x-amzn-RequestId: 1ffba607-ff3d-42c9-bdb9-78d1630c861d

{
"message": "Missing Authentication Token"
}
```

Start by running the provided Chalice app and requesting both `/` and `/hello` as shown. Trace the local route handling that produces the 403 response, then verify that an invalid route returns HTTP 404 while the valid route remains unchanged.

Written by the indexing model from the issue text.