Only warn about missing FAT image path when SDSUPPORT is enabled

[rhapsodyv]

We can try crate a fat fs volume automatic:

http://elm-chan.org/fsw/ff/doc/mkfs.html

[p3p]

nothing that fancy but I did add a button for creating an empty image yesterday https://github.com/p3p/MarlinSimUI/commit/e1319518304ff3c0deb0d28fb032b0071bdf2a17

[rhapsodyv]

hahahahah clever! now it's time for pizza! :-p