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 "Upload Picture" button, select {nav icon=picture, name=Edit Picture} in the sidebar, select `Custom: Choose Icon and Color...`, `Choose Background Color` and `Choose Icon`, then click the `Save Image` button. Also try to upload a custom image and look at the error message.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15720
Differential Revision: https://we.phorge.it/D25525
e2bec4c1f5