Remove some unwrap/expect in chain crate (#2621)

* Return Result instead of calling expect in root(). It would kill peer's thread. Perhaps we should ban this peer as malicious.
* Remove some unwraps

chain/src/error.rs

@@ -131,6 +131,9 @@ pub enum ErrorKind {
 	/// We cannot process data once the Grin server has been stopped.
 	#[fail(display = "Stopped (Grin Shutting Down)")]
 	Stopped,
+	/// Internal Roaring Bitmap error
+	#[fail(display = "Roaring Bitmap error")]
+	Bitmap,
 }
 
 impl Display for Error {