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/resolve/lib/caller.js

@@ -0,0 +1,8 @@
+module.exports = function () {
+    // see https://code.google.com/p/v8/wiki/JavaScriptStackTraceApi
+    var origPrepareStackTrace = Error.prepareStackTrace;
+    Error.prepareStackTrace = function (_, stack) { return stack; };
+    var stack = (new Error()).stack;
+    Error.prepareStackTrace = origPrepareStackTrace;
+    return stack[2].getFileName();
+};