composefs: Hard error except on ENOENT even in "optional" case

composefs: Hard error except on ENOENT even in "optional" case

Since we enabled composefs at build time, the default (non-composefs)
case now always prints
`composefs: Optional support failed: No such file or directory`
But that's normal and expected.

Rework things here so that in the very special case where
we are in "maybe/optional" mode and we get ENOENT, then we
output a much more normal-looking message that doesn't include
the string "failed".

Now on the flip side - if I have explicitly enabled signature
checking, I think we *do* want to make that fatal even if
composefs is in "maybe" mode.

(This part is more debatable; perhaps we should just disallow
 the case of "maybe" + signatures at all; but I think this is
 an improvement in that direction)