add error handler

Author: ben-lin

lib/flow.js

@@ -97,18 +97,42 @@ var Flow = function (){
 
 Flow.prototype = {
 
+/**
+ * Default error handler.
+ * @private
+ * @this {Flow}
+ * @param {Error} err Error to throw.
+ */
+  _error : function ( err ){
+    throw err;
+  },
+
+
+
+/**
+ * Check if the stack handler is the `next` handler.
+ * @private
+ * @this {Flow}
+ * @param {String} str The handler to be check.
+ */
   _is_next : function ( str ){
     return /self\[ '_' \+ next_type \]\.apply\( self, slice\.call\( arguments \)\);/g.test( str );
   },
 
+
+
 /**
  * Call the current series function and remove it.
  * @private
  * @this {Flow}
  * @param {Array} args Arguments to be passed to the current series function.
  */
   _run_series : function ( args ){
-    this._series.shift().apply( this, args );
+    try{
+      this._series.shift().apply( this, args );
+    }catch( err ){
+      this._error( err );
+    }
   },
 
 
@@ -145,7 +169,6 @@ Flow.prototype = {
  * @param {Array} args Arguments to be added to the current parallel function.
  */
   _run_parallel : function ( args ){
-
     var self                = this;
     var parallel_args       = this._parallel_args.shift();
     var args_form_last_task = args;
@@ -357,6 +380,12 @@ Flow.prototype = {
     return this;
   },
 
+  error : function ( callback ){
+    this._error = callback;
+
+    return this;
+  },
+
 
 
 /**