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/cmp.js

@@ -0,0 +1,23 @@
+var test = require('tape');
+var equal = require('../');
+
+test('equal', function (t) {
+    t.ok(equal(
+        { a : [ 2, 3 ], b : [ 4 ] },
+        { a : [ 2, 3 ], b : [ 4 ] }
+    ));
+    t.end();
+});
+
+test('not equal', function (t) {
+    t.notOk(equal(
+        { x : 5, y : [6] },
+        { x : 5, y : 6 }
+    ));
+    t.end();
+});
+
+test('nested nulls', function (t) {
+    t.ok(equal([ null, null, null ], [ null, null, null ]));
+    t.end();
+});