git-annex: clearer error if git-annex is missing

[kousu]

If git-annex is not installed on a system where gitea is running, the error messages are not at all clear that this is the problem.

Improve this. Make sure to cover:

• viewing the web UI
• connecting over ssh
• connecting over http
• when attempting to upload and to download

[kousu]

Alternately, maybe just refuse to run if git-annex is enabled but not found at boot.