bsdinstall: Fail nicely in jail target

[fernape]

Summary

If the directory is empty, then we fail with a message regarding mkdir in
which the empty directory can't be seen because it is not quoted.

Show a nice message so the user knows what is going on.

Test Plan

Apply patch and execute:
bsdinstall jail

[bapt]

small style remark, please address before pushing, other that that, lookgs good to me

usr.sbin/bsdinstall/scripts/jail

56

to be consistent with the rest of the code in this script please remove the { and }

[fernape]

Done with the suggested change.

Thanks for the review!