Improve error reporting from flatpak when low on disk space

Submitted by Bastien Nocera @hadess

Assigned to Richard Hughes @hughsie

Link to original bug (#786654)

Description

See https://github.com/flatpak/flatpak/issues/975 for the underlying problem

1. Fill up your home partition so that less than 3% of the space is left available
2. Try to update software in gnome-software

Aug 22 23:27:51 classic gnome-software-service.desktop[24211]: 21:27:51:0036 Gs failed to install org.gnome.Games.desktop: min-free-space-percent '3%' would be exceeded, 4.0 GB available

flatpak-0.9.7-5.fc27.x86_64 gnome-software-3.24.3-1.fc26.x86_64