fatal errors in test callbacks are not displayed (with fix)

Fatal errors in callbacks defined in the test functions are not displayed,
only the "test.done() not called" diagnostic is shown. It's useful to have
the error and line number too. (If not in a callback, the error is reported
as expected.)

To reproduce the issue, create a file t.js:

```
module.exports = {
test: function(t) {
var x;
setTimeout(function() {
x.a = 1;
}, 1);
}
}
```

Current nodeunit outputs:

```
% nodeunit t.js

t.js

FAILURES: Undone tests (or their setups/teardowns):
- test

To fix this, make sure all tests call test.done()
```

with the propsed fix it outputs:

```
% nodeunit t.js

t.js
Uncaught exception:
TypeError: Cannot set property 'a' of undefined
at null._onTimeout (/home/andras/node/t.js:5:21)
at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)

FAILURES: Undone tests (or their setups/teardowns):
- test

To fix this, make sure all tests call test.done()
```

The proposed fix:

```
diff --git a/lib/track.js b/lib/track.js
index 5af98ad..f699fce 100644
--- a/lib/track.js
+++ b/lib/track.js
@@ -27,6 +27,11 @@ exports.createTracker = function (on_exit) {
}
};

+ // AR: display errors in callbacks defined within the tests
+ process.on('uncaughtException', function(e) {
+ console.log("Uncaught exception:\n" + e.stack);
+ });
+
process.on('exit', function() {
on_exit = on_exit || exports.default_on_exit;
on_exit(tracker);
```

Commencez par lib/track.js et reproduisez le problème en utilisant l’exemple t.js du rapport. Comparez la sortie de nodeunit pour l’erreur du callback avec la sortie attendue, y compris la pile de l’exception non interceptée et le diagnostic undone-test existant ; l’issue est terminée lorsque l’erreur fatale du callback et le numéro de ligne sont affichés.

Rédigé par le modèle d'indexation à partir du texte de l'issue.