Convert NoMoreElementsException and EmptyQueueException to StateError.

lib/core/errors.dart

Index: lib/core/errors.dart
diff --git a/lib/core/errors.dart b/lib/core/errors.dart
index eca6fea8f426adfa811e263be69e811da3b20fac..9a16d83d2d2bd23964cd7fb7b1480f2619e0f74b 100644
--- a/lib/core/errors.dart
+++ b/lib/core/errors.dart
@@ -156,7 +156,7 @@ class NoSuchMethodError implements Error {
 class UnsupportedError implements Error {
   final String message;
   UnsupportedError(this.message);
-  String toString() => message;
+  String toString() => "Unsupported operation: message";
 }
 
 /**
@@ -168,7 +168,7 @@ class UnsupportedError implements Error {
 class StateError implements Error {
   final String message;
   StateError(this.message);
-  String toString() => message;
+  String toString() => "Bad state: message";
 }