[SeaBIOS] Clearer and more informative error messages

[Fred .]

"Boot failed: could not read the boot disk"
Happend when I started 'qemu' as just that without parameters.
So the reason it cant read it is that no such disk exists.

So it should tell me "could not find any disk" instead of "could not
read the boot disk" which suggest it found one but was unable to read
it.