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/left-pad/index.d.ts

@@ -0,0 +1,9 @@
+// Type definitions for left-pad 1.2.0
+// Project: https://github.com/stevemao/left-pad
+// Definitions by: Zlatko Andonovski, Andrew Yang, Chandler Fang and Zac Xu
+
+declare function leftPad(str: string|number, len: number, ch?: string|number): string;
+
+declare namespace leftPad { }
+
+export = leftPad;