guix pull -l fails on fresh user profiles

> Switch to a new user, or simply log in as 'root' if you've never "guix

> pull"ed with it, then

>

> # guix pull -l

> Backtrace:

> 8 (apply-smob/1 #<catch-closure 11508e0>)

> In ice-9/boot-9.scm:

> 705:2 7 (call-with-prompt _ _ #<procedure default-prompt-handle…>)

> In ice-9/eval.scm:

> 619:8 6 (_ #(#(#<directory (guile-user) 11d2140>)))

> In guix/ui.scm:

> 1644:12 5 (run-guix-command _ . _)

> In ice-9/boot-9.scm:

> 829:9 4 (catch srfi-34 #<procedure 2e24be0 at guix/ui.scm:622:…> …)

> 829:9 3 (catch system-error #<procedure 2e24c80 at guix/script…> …)

> 829:9 2 (catch git-error #<procedure 2e24b40 at guix/scripts/p…> …)

> 829:9 1 (catch system-error #<procedure 2e28c40 at guix/script…> …)

> In unknown file:

> 0 (raise #<condition &profile-not-found-error [profile: "…>)

>

> ERROR: In procedure raise:

> Wrong type (expecting exact integer): #<condition &profile-not-found-error [profile: "/var/guix/profiles/per-user/root/current-guix"] 2e1e8c0>

> I think we should make `guix pull -l' return a value that can be used

> meaningfully in deployment scripts. For instance, a non-zero

> exit code could mean this is a fresh user profile.