improved error handling

[part-time-githubber]

No description provided.

[freewil]

I'll have to take a closer look at how this streaming func is used, but initial thoughts:

can you rename stderrData to error, rename data to buf and initialize error as an empty string and then change the push so that it appends to error so it is consistent with this:

node-gpg/lib/spawnGPG.js

Lines 28 to 38 in 510c58c

	var error = '';
	var gpg = spawnIt(gpgArgs, cb);
	gpg.stdout.on('data', function (buf){
	buffers.push(buf);
	buffersLength += buf.length;
	});
	gpg.stderr.on('data', function(buf){
	error += buf.toString('utf8');
	});