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/uglify-js/tools/exit.js

@@ -0,0 +1,15 @@
+// workaround for tty output truncation upon process.exit()
+var exit = process.exit;
+process.exit = function() {
+    var args = [].slice.call(arguments);
+    process.once("uncaughtException", function() {
+        (function callback() {
+            if (process.stdout.bufferSize || process.stderr.bufferSize) {
+                setImmediate(callback);
+            } else {
+                exit.apply(process, args);
+            }
+        })();
+    });
+    throw exit;
+};