Got rid of static/templates/no_search_result.html

The "Fulltext search unavailable" error page is now generated using the
static/templates/error.html template. Also added two test cases checking
that error page.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f