Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/asynckit/lib/terminator.js

@@ -0,0 +1,29 @@
+var abort = require('./abort.js')
+  , async = require('./async.js')
+  ;
+
+// API
+module.exports = terminator;
+
+/**
+ * Terminates jobs in the attached state context
+ *
+ * @this  AsyncKitState#
+ * @param {function} callback - final callback to invoke after termination
+ */
+function terminator(callback)
+{
+  if (!Object.keys(this.jobs).length)
+  {
+    return;
+  }
+
+  // fast forward iteration index
+  this.index = this.size;
+
+  // abort jobs
+  abort(this);
+
+  // send back results we have so far
+  async(callback)(null, this.results);
+}