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/throat/index.js.flow

@@ -0,0 +1,7 @@
+// @flow
+
+declare function throat<TResult, TFn: (...args: Array<any>) => Promise<TResult>>(size: number, fn: TFn): TFn;
+declare function throat<TResult, TFn: (...args: Array<any>) => Promise<TResult>>(fn: TFn, size: number): TFn;
+declare function throat(size: number): <TResult>(fn: () => Promise<TResult>) => Promise<TResult>;
+
+module.exports = throat;