Introduction
                        
                            Retry and RetryWhen can be used to attempt to recover Observables that might have errors in their stream.
                         
                        Syntax
- .retry(n: number): Observable
- n: retry will attempt the source Observable this many times.
 
 
- .retryWhen(receives: notificationHandler, the: scheduler): Observable
- receives: an Observable of notifications which the use can complete or error.
- If the 'receives' Observable returns cleanly
(completes) the source Observable will be reattempted.
 
- If the 'receives' Observable returns an error, the source Observable is
aborted.
 
 
- scheduler: The source Observable is subscribed to this scheduler.