test(Device): Add tests for Device flow

src/authenticator_delegate.rs

@@ -56,6 +56,8 @@ pub enum PollError {
     AccessDenied,
     /// Indicates that too many attempts failed.
     TimedOut,
+    /// Other type of error.
+    Other(String)
 }
 
 impl fmt::Display for PollError {
@@ -65,6 +67,7 @@ impl fmt::Display for PollError {
             PollError::Expired(ref date) => writeln!(f, "Authentication expired at {}", date),
             PollError::AccessDenied => "Access denied by user".fmt(f),
             PollError::TimedOut => "Timed out waiting for token".fmt(f),
+            PollError::Other(ref s) => format!("Unknown server error: {}", s).fmt(f)
         }
     }
 }