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/fp/_convertBrowser.js

@@ -0,0 +1,18 @@
+var baseConvert = require('./_baseConvert');
+
+/**
+ * Converts `lodash` to an immutable auto-curried iteratee-first data-last
+ * version with conversion `options` applied.
+ *
+ * @param {Function} lodash The lodash function to convert.
+ * @param {Object} [options] The options object. See `baseConvert` for more details.
+ * @returns {Function} Returns the converted `lodash`.
+ */
+function browserConvert(lodash, options) {
+  return baseConvert(lodash, lodash, options);
+}
+
+if (typeof _ == 'function' && typeof _.runInContext == 'function') {
+  _ = browserConvert(_.runInContext());
+}
+module.exports = browserConvert;