Replace obsolete _error method.

Was removed in 3cd2196.
Fixes #33

Author: RubenVerborgh

lib/sparql/UnionIterator.js

@@ -16,7 +16,7 @@ function UnionIterator(sources, options) {
     // Create event listeners
     var self = this;
     function fillBuffer()     { self._fillBuffer(); }
-    function emitError(error) { self._error(error); }
+    function emitError(error) { self.emit('error', error); }
 
     // Add all readable sources and listen to their events
     for (var i = 0, l = sources.length; i < l; i++) {