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/jest-pnp-resolver/index.d.ts

@@ -0,0 +1,10 @@
+type JestResolverOptions = {
+  basedir: string;
+  defaultResolver: (request: string, opts: any) => string,
+  extensions?: Array<string>,
+};
+
+export default function resolve(
+  request: string,
+  options: JestResolverOptions,
+): string;