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/is-generator-fn/index.d.ts

@@ -0,0 +1,24 @@
+declare const isGeneratorFn: {
+	/**
+	Check if something is a generator function.
+
+	@example
+	```
+	import isGeneratorFn = require('is-generator-fn');
+
+	isGeneratorFn(function * () {});
+	//=> true
+
+	isGeneratorFn(function () {});
+	//=> false
+	```
+	*/
+	(value: unknown): value is GeneratorFunction;
+
+	// TODO: Remove this for the next major release, refactor the whole definition to:
+	// declare function isGeneratorFn(value: unknown): value is GeneratorFunction;
+	// export = isGeneratorFn;
+	default: typeof isGeneratorFn;
+};
+
+export = isGeneratorFn;