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.js

@@ -0,0 +1,46 @@
+'use strict';
+
+const fs = require('fs');
+const promisify = require('util.promisify');
+
+const promisiedFsRealpath = promisify(fs.realpath);
+
+function realpath(filepath) {
+  if (typeof fs.realpath.native === 'function') {
+    return promisify(fs.realpath.native)(filepath);
+  }
+  const fsBinding = process.binding('fs');
+
+  if (fsBinding.realpath) {
+    return new Promise((resolve, reject) => {
+      try {
+        resolve(fsBinding.realpath(filepath, 'utf8'));
+      } catch (e) {
+        reject(e);
+      }
+    });
+  }
+
+  return promisiedFsRealpath(filepath);
+}
+
+function realpathSync(filepath) {
+  if (typeof fs.realpathSync.native === 'function') {
+    return fs.realpathSync.native(filepath);
+  }
+
+  const fsBinding = process.binding('fs');
+
+  if (fsBinding.realpath) {
+    try {
+      return fsBinding.realpath(filepath, 'utf8');
+    } catch (err) {
+      /* Probably RAM-disk on windows. */
+    }
+  }
+
+  return fs.realpathSync(filepath);
+}
+
+module.exports = realpath;
+module.exports.sync = realpathSync;