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/_escapeHtmlChar.js

@@ -0,0 +1,21 @@
+var basePropertyOf = require('./_basePropertyOf');
+
+/** Used to map characters to HTML entities. */
+var htmlEscapes = {
+  '&': '&',
+  '<': '&lt;',
+  '>': '&gt;',
+  '"': '&quot;',
+  "'": '&#39;'
+};
+
+/**
+ * Used by `_.escape` to convert characters to HTML entities.
+ *
+ * @private
+ * @param {string} chr The matched character to escape.
+ * @returns {string} Returns the escaped character.
+ */
+var escapeHtmlChar = basePropertyOf(htmlEscapes);
+
+module.exports = escapeHtmlChar;