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/realpath-native/index.d.ts

@@ -0,0 +1,6 @@
+declare function realpath(filepath: string): string;
+declare namespace realpath {
+  var sync: typeof realpathSync;
+}
+declare function realpathSync(filepath: string): string;
+export = realpath;