robots.txt 404 should be different

[sinnlosername]

• Gitea version (or commit ref): 1.1.2 built with: bindata, sqlite
• Operating system: Ubuntu 16.04.2 LTS
• Database (use [x]):
  • MySQL
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant
  • Site is down

Description

If there is no custom robots.txt, it will return an empty 404 instead of the default gitea 404 error.