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/pn/test/load.js

@@ -0,0 +1,14 @@
+// Just verify that all of the packages can be loaded without errors.
+var fs = require('fs'), path = require('path');
+// if running on older node, ensure that es6-shim is loaded first
+if (/^v0.10/.test(process.version)) { require('es6-shim'); }
+
+describe("Package load tests", function() {
+    fs.readdirSync(path.join(__dirname, '..')).forEach(function(file) {
+        if (/^_/.test(file) || !/\.js$/.test(file)) { return; }
+        var pkgname = file.replace(/\.js$/, '');
+        it(pkgname+' should load', function() {
+            require('../'+pkgname);
+        });
+    });
+});