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/_castSlice.js

@@ -0,0 +1,18 @@
+var baseSlice = require('./_baseSlice');
+
+/**
+ * Casts `array` to a slice if it's needed.
+ *
+ * @private
+ * @param {Array} array The array to inspect.
+ * @param {number} start The start position.
+ * @param {number} [end=array.length] The end position.
+ * @returns {Array} Returns the cast slice.
+ */
+function castSlice(array, start, end) {
+  var length = array.length;
+  end = end === undefined ? length : end;
+  return (!start && end >= length) ? array : baseSlice(array, start, end);
+}
+
+module.exports = castSlice;