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/deep-is/test/NaN.js

@@ -0,0 +1,16 @@
+var test = require('tape');
+var equal = require('../');
+
+test('NaN and 0 values', function (t) {
+    t.ok(equal(NaN, NaN));
+    t.notOk(equal(0, NaN));
+    t.ok(equal(0, 0));
+    t.notOk(equal(0, 1));
+    t.end();
+});
+
+
+test('nested NaN values', function (t) {
+    t.ok(equal([ NaN, 1, NaN ], [ NaN, 1, NaN ]));
+    t.end();
+});