Fix crash in RMBingSource when the user is offline

These changes fix a crash in `RMBingSource` when the user is offline. 

Also included in the pull request are `NSError` parameters which are changed from `nil` to `NULL` (https://github.com/mapbox/mapbox-ios-sdk/issues/287), and a typo fix in a `typedef` declaration in `RMBingSource.h`