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/isarray/test.js

@@ -0,0 +1,20 @@
+var isArray = require('./');
+var test = require('tape');
+
+test('is array', function(t){
+  t.ok(isArray([]));
+  t.notOk(isArray({}));
+  t.notOk(isArray(null));
+  t.notOk(isArray(false));
+
+  var obj = {};
+  obj[0] = true;
+  t.notOk(isArray(obj));
+
+  var arr = [];
+  arr.foo = 'bar';
+  t.ok(isArray(arr));
+
+  t.end();
+});
+