Added error handler for stdin and stdout.

[freezy]

I have unhandled stream errors in my tests recently. Something to do with concurrency, the PNG is deleted just before or while launching pngquant, which breaks the whole test suite because the error isn't treatable.

This patch properly adds an error handler on the process' stdin and stdout, making these kind of errors treatable for the caller.

[papandreou]

Thanks for this. Is it triggered by an external process such as tmpreaper removing the temporary files while the operation is running?

Sounds like a fix that should be applied to https://github.com/papandreou/node-optipng, https://github.com/papandreou/node-pngcrush, and https://github.com/papandreou/node-inkscape as well. Can you think of a way to add an automated test for it?

[freezy]

It's triggered by the after() method of my Mocha test, which cleans up files. Since my setup processes images asynchronously with a message queue, it is launched even though the files might have been deleted already.

I'll look into a test case later. I haven't looked at your other projects, but if you're using the same approach, it probably needs fixing there as well.

[papandreou]

I'll take a chance and merge this in its current state. A test would be appreciated if you can come up with one later.

[papandreou]

Released in 0.4.0.

[freezy]

Thanks. Still need to finish some other stuff first, but the mental note is still in my head :)