Return ERR_UNAVAILABLE if native dialog is not supported

[timothyqiu]

Otherwise, there's no way to tell whether the dialog actually pops up (except for hard-coding supported platforms).

[bruvzg]

The change looks OK.

But it's probably something that should be also checkable via has_feature. There's already FEATURE_NATIVE_DIALOG for the native file dialogs, it probably should be split into multiple options to allow checking for each type of dialog.

[timothyqiu]

Because these functions return Error, they could technically still fail to show the dialog for other reasons on display servers that do have an implementation. In practice, I find it reasonable to handle all "failure of opening a native dialog" situation the same way :P

[akien-mga]

Thanks!