Error handler for rewrite in URLRewriteStream

[chmanie]

If there's an error in the transform function (called replacer) it just bubbles up to the highest scope. I'd like to have more control over errors thrown there so I'd propose to wrap the actual rewriting into a try-catch and let the stream emit an error event when it is caught. This would be the expected behaviour when dealing with a stream (at least for me).

I already prepared a PR, let me know if this is acceptable for you.

[nzakas]

Sounds good.