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/_isMaskable.js

@@ -0,0 +1,14 @@
+var coreJsData = require('./_coreJsData'),
+    isFunction = require('./isFunction'),
+    stubFalse = require('./stubFalse');
+
+/**
+ * Checks if `func` is capable of being masked.
+ *
+ * @private
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `func` is maskable, else `false`.
+ */
+var isMaskable = coreJsData ? isFunction : stubFalse;
+
+module.exports = isMaskable;