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/lodash/_baseLodash.js

@@ -0,0 +1,10 @@
+/**
+ * The function whose prototype chain sequence wrappers inherit from.
+ *
+ * @private
+ */
+function baseLodash() {
+  // No operation performed.
+}
+
+module.exports = baseLodash;