Show better error messages in network failures.

utils/pub/io.dart

Index: utils/pub/io.dart
diff --git a/utils/pub/io.dart b/utils/pub/io.dart
index 8b632e3b47320f2d35c84910c0c2165502be8f2c..26647c86d54c041e6ce96628519032f269c07d65 100644
--- a/utils/pub/io.dart
+++ b/utils/pub/io.dart
@@ -712,6 +712,8 @@ class PubHttpException implements Exception {
   final String reason;
 
   const PubHttpException(this.statusCode, this.reason);
+
+  String toString() => 'HTTP error $statusCode: $reason';
 }
 
 /**
@@ -721,6 +723,8 @@ class TimeoutException implements Exception {
   final String message;
 
   const TimeoutException(this.message);
+
+  String toString() => message;
 }
 
 /**