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/_unescapeHtmlChar.js

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