gcalctool: Two strings missed for translation now added

[Robert Ancell <robert ancell gmail com>]

As reported by Kjartan Maraas [1] in gcalctool there are two strings
that were accidentally not marked for translation:
"Error: the number of periods must be positive"
"Calculating"

These have now been marked in the gnome-3-0 branch.

[1] https://bugzilla.gnome.org/show_bug.cgi?id=646093