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/_baseToNumber.js

@@ -0,0 +1,24 @@
+var isSymbol = require('./isSymbol');
+
+/** Used as references for various `Number` constants. */
+var NAN = 0 / 0;
+
+/**
+ * The base implementation of `_.toNumber` which doesn't ensure correct
+ * conversions of binary, hexadecimal, or octal string values.
+ *
+ * @private
+ * @param {*} value The value to process.
+ * @returns {number} Returns the number.
+ */
+function baseToNumber(value) {
+  if (typeof value == 'number') {
+    return value;
+  }
+  if (isSymbol(value)) {
+    return NAN;
+  }
+  return +value;
+}
+
+module.exports = baseToNumber;