Improve reporting of JavaScript stack trace

[pdawyndt]

Example: submission 13859481

In this example submission, stack traces are reported as

AssertionError [ERR_ASSERTION]: ongeldig bericht
    at <code>:67:4
<code>:76
    at <code>:76:5

with duplicate reporting of line 76 and broken indentation.

[niknetniko]

@pdawyndt Do you still remember which submission it was? The link goes to the exercise.

[pdawyndt]

I updated the ID and the link of the example submission (was one of my own submission, so easy enough to find it back).