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/_hashClear.js

@@ -0,0 +1,15 @@
+var nativeCreate = require('./_nativeCreate');
+
+/**
+ * Removes all key-value entries from the hash.
+ *
+ * @private
+ * @name clear
+ * @memberOf Hash
+ */
+function hashClear() {
+  this.__data__ = nativeCreate ? nativeCreate(null) : {};
+  this.size = 0;
+}
+
+module.exports = hashClear;