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/ajv/lib/compile/ucs2length.js

@@ -0,0 +1,20 @@
+'use strict';
+
+// https://mathiasbynens.be/notes/javascript-encoding
+// https://github.com/bestiejs/punycode.js - punycode.ucs2.decode
+module.exports = function ucs2length(str) {
+  var length = 0
+    , len = str.length
+    , pos = 0
+    , value;
+  while (pos < len) {
+    length++;
+    value = str.charCodeAt(pos++);
+    if (value >= 0xD800 && value <= 0xDBFF && pos < len) {
+      // high surrogate, and there is a next character
+      value = str.charCodeAt(pos);
+      if ((value & 0xFC00) == 0xDC00) pos++; // low surrogate
+    }
+  }
+  return length;
+};