Warn when skipping installations

When we fail to parse an installation, emit some
warnings that give the user a chance to figure out
what is going wrong. I ran into the missing quotes
issue myself when I first tried to create a custom
installation.

Closes: #1939
Approved by: alexlarsson
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