Raise validation error on duplicate routes

[jamesls]

Routing is based solely on URL path. This can cause issues that are hard to troubleshoot. Consider

from chalice import Chalice

app = Chalice(app_name='quiet-venv')


@app.route('/foo', methods=['GET'])
def index():
    return {'hello': 'world'}

@app.route('/foo', methods=['PUT'])
def index_put():
    return {'hello': 'world'}

The last route will override the previous route because they have the same route key, /foo. This is almost never what someone would want. Instead what should happen here is a validation error saying that you've created a duplicate route.

There is a case to be made to actually support this in the future (route based on path as well as method), but for time being this should be a validation error.