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);
```

Beginne mit lib/track.js und reproduziere das Problem anhand des t.js-Beispiels aus dem Bericht. Vergleiche die Ausgabe von nodeunit für den Callback-Fehler mit der erwarteten Ausgabe, einschließlich des Stacks der nicht abgefangenen Exception und der vorhandenen undone-test-Diagnose; das Issue ist erledigt, wenn der fatale Callback-Fehler und die Zeilennummer angezeigt werden.

Vom Indexierungsmodell aus dem Issue-Text verfasst.