Fix exception name

Author: brianquinlan

pkgs/cupertino_http/example/integration_test/ns_error_client_exception.dart

@@ -26,7 +26,7 @@ void main() {
               .having(
                   (e) => e.toString(),
                   'toString()',
-                  'NSErrorClientExceptions: A server with the specified '
+                  'NSErrorClientException: A server with the specified '
                       'hostname could not be found. '
                       '[domain=NSURLErrorDomain, code=-1003], '
                       'uri=http://doesnotexist/')));

pkgs/cupertino_http/lib/src/cupertino_client.dart

@@ -27,7 +27,7 @@ class NSErrorClientExceptions extends ClientException {
   @override
   String toString() {
     final b = StringBuffer(
-        'NSErrorClientExceptions: ${error.localizedDescription.toDartString()} '
+        'NSErrorClientException: ${error.localizedDescription.toDartString()} '
         '[domain=${error.domain.toDartString()}, code=${error.code}]');
 
     if (uri != null) {