Fixed double TT rendering on error

If a template produced a rendering error in process() but did not produce one
when called without stash variables, the Mojo::Exception would be built
incorrectly, leading to an exception that masked the original error.

t/tt_plugin_lite_app.t

@@ -15,7 +15,7 @@ BEGIN { $ENV{MOJO_TMPDIR} ||= File::Temp::tempdir }
 # Make sure sockets are working
 plan skip_all => 'working sockets required for this test!'
   unless Mojo::IOLoop->new->generate_port;
-plan tests => 3;
+plan tests => 5;
 
 # Leela: OK, this has gotta stop. I'm going to remind Fry of his humanity the way only a woman can.
 # Farnsworth: You're going to do his laundry?
@@ -30,13 +30,18 @@ plugin 'tt_renderer';
 app->log->level('error');
 
 # GET /
-get '/' => 'index';
+get '/'     => 'index';
+get '/blow' => sub {
+    shift->render(template => 'conditional-exception', do_process => 1);
+};
+
 
 my $t = Test::Mojo->new;
 
 # Simple TT template
 $t->get_ok('/')->status_is(200)
   ->content_like(qr/test123456/);
+$t->get_ok('/blow')->content_like(qr/file error - doesnotexist.tt: No such file or directory/);
 
 eval "
   use Devel::Cycle 'find_cycle';