Improve error message

[flaviojs]

"error: expected }

I got this error and failed to find the problem with a manual inspection of the text.
With a bunch of "cut parts of text"-assemble-"paste back" cycles I managed to pinpoint the troublesome lines.

If the error also showed the location of the { that it was trying to match, then I could have fixed the troublesome lines with ease.

[hlorenzi]

That's certainly a good idea! I'll take a look and see if it's any easy to implement in the current architecture.