TravelTime sdk should behave better when passing wrong URI

[bartosz-witkowski]

Currently when passing a wrong URI the error is quite opaque, e.g constructing TravelTimeSdk with the following URI new URI("http://api.traveltimeapp.com/v4) (not the absence of a trailing slash after v4) will return errors from sdk.send which are non obvious:

Something went wrong when parsing json response: Unrecognized token 'default': was expecting (JSON String, Number, Array, Object or token 'null', 'true' or 'false')
 at [Source: (String)"default backend - 404"; line: 1, column: 8]