Custom /icons folder bug

[Myrathi]

There's a quirk where MultiMC won't accept any custom icon files as valid unless there's a default icon override in the folder, already, and the override is the first one to be loaded.

As it stands, the best default icon to override (due to sort order) is: brick

The tests in this ticket assume you've already created the /icons subfolder inside your MultiMC folder.

Broken example 1

• Your /icons folder is empty.
• Add an image that is not an override of an existing icon: e.g. test.png
• Run MultiMC
  • Error!

Broken example 2:

• Add a default icon override to your /icons folder: e.g. brick.png
• Add an image that does not override -and- sorts before the default filename: e.g. aardvark.jpg
• Run MultiMC
  • Error!

Working example:

• Add a default icon override to your /icons folder: e.g. brick.png
• Add an image that does not override -and- sorts after the default filename: e.g. donkey.png
• Run MultiMC
  • No error.

With the working example, any number of extra images can be added to the /icons folder, so long as the first sorted filename is always a default icon.