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/lcid/index.js

@@ -0,0 +1,23 @@
+'use strict';
+const invertKv = require('invert-kv');
+const all = require('./lcid.json');
+
+const inverted = invertKv(all);
+
+exports.from = lcidCode => {
+	if (typeof lcidCode !== 'number') {
+		throw new TypeError('Expected a number');
+	}
+
+	return inverted[lcidCode];
+};
+
+exports.to = localeId => {
+	if (typeof localeId !== 'string') {
+		throw new TypeError('Expected a string');
+	}
+
+	return all[localeId];
+};
+
+exports.all = all;