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/fast-deep-equal/index.d.ts

@@ -0,0 +1,4 @@
+declare module 'fast-deep-equal' {
+    const equal: (a: any, b: any) => boolean;
+    export = equal;
+}