Return result error from SingleResult before nil registry check

Author: edaniels

mongo/single_result.go

@@ -33,6 +33,9 @@ type SingleResult struct {
 // will be returned. If there were no returned documents, ErrNoDocuments is
 // returned.
 func (sr *SingleResult) Decode(v interface{}) error {
+	if sr.err != nil {
+		return sr.err
+	}
 	if sr.reg == nil {
 		return bson.ErrNilRegistry
 	}