handle output from stderr. closes #11

index.js

@@ -222,11 +222,24 @@ ImageMagick.prototype = {
     var stdout = proc.stdout;
     stdout.on('error', this.onerror);
     stdout.pipe(this.out);
+
+    var stderr = proc.stderr;
+    stderr.on('data', this.onerror);
+    stderr.on('error', this.onerror);
 
     this.emit('spawn', proc);
   },
 
+  /**
+   * Re-emit errors
+   *
+   * @param {Error|Buffer} err
+   * @api private
+   */
+
   onerror: function (err) {
+    if (!isError(err)) err = new Error(err);
+    if (!this.listeners('error')) throw err;
     this.emit('error', err);
   }
 };

test/index.js

@@ -28,6 +28,16 @@ describe('im()', function () {
     });
   });
 
+  it('should emit errors from stderr', function (done) {
+    var img = im();
+    img.write('invalid data');
+    img.end();
+    img.on('error', function (err) {
+      assert(/^convert\:/.test(err.message));
+      done();
+    });
+  });
+
   describe('.from()', function () {
     it('should read from the given path', function (done) {
       var img = im().from(__dirname + '/test.jpg');