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/p-is-promise/index.d.ts

@@ -0,0 +1,23 @@
+declare const pIsPromise: {
+	/**
+	Check if `input` is a ES2015 promise.
+
+	@param input - Value to be checked.
+
+	@example
+	```
+	import isPromise = require('p-is-promise');
+
+	isPromise(Promise.resolve('🦄'));
+	//=> true
+	```
+	*/
+	(input: unknown): input is Promise<unknown>;
+
+	// TODO: Remove this for the next major release, refactor the whole definition to:
+	// declare function pIsPromise(input: unknown): input is Promise<unknown>;
+	// export = pIsPromise;
+	default: typeof pIsPromise;
+};
+
+export = pIsPromise;