R1:e2bec4c1f58b

Improve messages when no image formats are supported (due to GD not installed)

Summary:
When GD is not installed, trying to set a custom image for a project/blog/repository/user/etc displays unhelpful error messages (`This server only supports these image formats: .` and `Supported formats: `) due to the array of supported image formats being empty.

Display clearer messages instead.

Closes T15720

Test Plan: Do not have php-gd installed, go to `/project/manage/1/`, take a look at the string below the…