show message for when api returns no buses

[EddyIonescu]

it's possible to choose a time for which we don't have any data - the client has no way of knowing if that's the case in advance though.

What the API does now: returns an empty list of buses
What the client does now: nothing (ie. renders the empty list of buses)
What the client should do: same as now - but show an error message (either a pop-up or in a red-textfield in the Time section)