common: Mark an error message as translatable

All the other error messages in that file are.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #2609
Approved by: matthiasclasen
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