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/istanbul-lib-instrument/dist/constants.js

@@ -0,0 +1,22 @@
+"use strict";
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+exports.MAGIC_VALUE = exports.MAGIC_KEY = exports.SHA = void 0;
+
+var _crypto = require("crypto");
+
+var _semver = require("semver");
+
+var _package = require("../package.json");
+
+// function to use for creating hashes
+const SHA = 'sha1'; // name of coverage data magic key
+
+exports.SHA = SHA;
+const MAGIC_KEY = '_coverageSchema'; // name of coverage data magic value
+
+exports.MAGIC_KEY = MAGIC_KEY;
+const MAGIC_VALUE = (0, _crypto.createHash)(SHA).update(_package.name + '@' + (0, _semver.major)(_package.version)).digest('hex');
+exports.MAGIC_VALUE = MAGIC_VALUE;