repo: Reword min-free-space-size option's error strings
repo: Reword min-free-space-size option's error strings

It is important that we use user-friendly error strings. The reason
being error strings are seen by users such as in GNOME Software's
error banner.

Closes: #1671
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree