Only send error if we have actual error stuff.

When using browservefy over a network the output from 

`stream.pipe(resp)` and `bfy.stderr.on('end', function () { bfyerror() })`
both write to the resp and write mixed / invalid javascript.

This is currently patched to assume that if something is written to `stderr` then
nothing is written to `stdout` so that works safely.

If something is written to both then they probably won't merge cleanly.

You should put some network latency in it to actual notice the broken merging.