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/_createRelationalOperation.js

@@ -0,0 +1,20 @@
+var toNumber = require('./toNumber');
+
+/**
+ * Creates a function that performs a relational operation on two values.
+ *
+ * @private
+ * @param {Function} operator The function to perform the operation.
+ * @returns {Function} Returns the new relational operation function.
+ */
+function createRelationalOperation(operator) {
+  return function(value, other) {
+    if (!(typeof value == 'string' && typeof other == 'string')) {
+      value = toNumber(value);
+      other = toNumber(other);
+    }
+    return operator(value, other);
+  };
+}
+
+module.exports = createRelationalOperation;