Ensure existence of repository icon if specified
Currently, a new repository includes a repository icon like so:
{
"repo": {
"icon": "fdroid-icon.png"
}
}
This isn't really used somewhere at the moment, but will be used in repomaker.
When the repository specifies an icon like that, it should also exist in the filesystem. I therefore suggest that fdroidserver throws an error or at least a warning when publishing a repository with a pointer to a non-existent icon.