Add "event not found" page

**Describe the bug**
There is no 404 indication page.

**Expected behavior**
When an event is not found or a link isn't right, the application should throw a `404 Not Found Error`

**Screenshots**
N/A

**Environment (please complete the following information):**

- Platform: both
- OS: all
- Browser all
- Version v0.1.2

**Additional context**
:p