Add some information to error message

I had to put this change in to spot where my typo was, so thought I would make a PR

lib/async.js

@@ -575,7 +575,7 @@
             var dep;
             while (len--) {
                 if (!(dep = tasks[requires[len]])) {
-                    throw new Error('Has inexistant dependency');
+                    throw new Error('Has inexistant dependency in ' + requires.join(', '));
                 }
                 if (_isArray(dep) && _indexOf(dep, k) >= 0) {
                     throw new Error('Has cyclic dependencies');