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/ts-jest/dist/util/jsonable-value.js

@@ -0,0 +1,34 @@
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+var json_1 = require("./json");
+var JsonableValue = (function () {
+    function JsonableValue(value) {
+        this.value = value;
+    }
+    Object.defineProperty(JsonableValue.prototype, "value", {
+        get: function () {
+            return this._value;
+        },
+        set: function (value) {
+            this._value = value;
+            this._serialized = json_1.stringify(value);
+        },
+        enumerable: true,
+        configurable: true
+    });
+    Object.defineProperty(JsonableValue.prototype, "serialized", {
+        get: function () {
+            return this._serialized;
+        },
+        enumerable: true,
+        configurable: true
+    });
+    JsonableValue.prototype.valueOf = function () {
+        return this._value;
+    };
+    JsonableValue.prototype.toString = function () {
+        return this._serialized;
+    };
+    return JsonableValue;
+}());
+exports.JsonableValue = JsonableValue;