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/_baseMean.js

@@ -0,0 +1,20 @@
+var baseSum = require('./_baseSum');
+
+/** Used as references for various `Number` constants. */
+var NAN = 0 / 0;
+
+/**
+ * The base implementation of `_.mean` and `_.meanBy` without support for
+ * iteratee shorthands.
+ *
+ * @private
+ * @param {Array} array The array to iterate over.
+ * @param {Function} iteratee The function invoked per iteration.
+ * @returns {number} Returns the mean.
+ */
+function baseMean(array, iteratee) {
+  var length = array == null ? 0 : array.length;
+  return length ? (baseSum(array, iteratee) / length) : NAN;
+}
+
+module.exports = baseMean;