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/_baseConforms.js

@@ -0,0 +1,18 @@
+var baseConformsTo = require('./_baseConformsTo'),
+    keys = require('./keys');
+
+/**
+ * The base implementation of `_.conforms` which doesn't clone `source`.
+ *
+ * @private
+ * @param {Object} source The object of property predicates to conform to.
+ * @returns {Function} Returns the new spec function.
+ */
+function baseConforms(source) {
+  var props = keys(source);
+  return function(object) {
+    return baseConformsTo(object, source, props);
+  };
+}
+
+module.exports = baseConforms;