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/_isMasked.js

@@ -0,0 +1,20 @@
+var coreJsData = require('./_coreJsData');
+
+/** Used to detect methods masquerading as native. */
+var maskSrcKey = (function() {
+  var uid = /[^.]+$/.exec(coreJsData && coreJsData.keys && coreJsData.keys.IE_PROTO || '');
+  return uid ? ('Symbol(src)_1.' + uid) : '';
+}());
+
+/**
+ * Checks if `func` has its source masked.
+ *
+ * @private
+ * @param {Function} func The function to check.
+ * @returns {boolean} Returns `true` if `func` is masked, else `false`.
+ */
+function isMasked(func) {
+  return !!maskSrcKey && (maskSrcKey in func);
+}
+
+module.exports = isMasked;