Default 404 error page.

[shoebpate1]

Design a 404 not found page for invalid URLs, should be consistent with current UI.

[shoebpate1]

@Emkiller As you are now done with other task.Take this simple one as it involves minimum dive into nodejs backend.
For example of 404 page, have a look at https://github.com/yhiiioooogggffgjjdgv

[mgautam98]

PR #16 closes this issue.