Show warning when a trip exceeds the max walk distance

[barbeau]

Reported by user on the OTP dev list:
https://groups.google.com/d/msg/opentripplanner-users/ZjG49IwIvgo/U9IpGA_ABwAJ

The max walk parameter can currently be set via the Settings. The default OTP server behavior is to return an intinerary, even if it exceeds the max walk parameter. So, we're currently returning trips to the user that exceeds their preferred max walk without letting them know this.

We should show a warning to the user when the walk distance of the trip exceeds the walk distance parameter.

Here's an example of how this warning looks in the Leaflet UI:

[barbeau]

There is a field in the server response that we can use to know if the walk limit was exceeded:

<itineraries>
...
<walkLimitExceeded>true</walkLimitExceeded>