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/_baseAssignValue.js

@@ -0,0 +1,25 @@
+var defineProperty = require('./_defineProperty');
+
+/**
+ * The base implementation of `assignValue` and `assignMergeValue` without
+ * value checks.
+ *
+ * @private
+ * @param {Object} object The object to modify.
+ * @param {string} key The key of the property to assign.
+ * @param {*} value The value to assign.
+ */
+function baseAssignValue(object, key, value) {
+  if (key == '__proto__' && defineProperty) {
+    defineProperty(object, key, {
+      'configurable': true,
+      'enumerable': true,
+      'value': value,
+      'writable': true
+    });
+  } else {
+    object[key] = value;
+  }
+}
+
+module.exports = baseAssignValue;