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);
```

Comienza con lib/track.js y reproduce el problema usando el ejemplo t.js del informe. Compara la salida de nodeunit para el error del callback con la salida esperada, incluida la pila de la excepción no capturada y el diagnóstico undone-test existente; el issue está resuelto cuando se muestran el error fatal del callback y el número de línea.

Escrito por el modelo de indexación a partir del texto del issue.