strset: set errno to ENOENT even if we return NULL.

strset: set errno to ENOENT even if we return NULL.

It seems redundant: strset_test() and strset_clear() can only return NULL
when the string is not a member.  However, it became clear in writing
ccan/tsort that it's much more convenient for callers if we set errno
in this case too, so they can pass it up.