Make the error dialog a bit less dire

Apologize first, and don't throw low-level error messages into
the poor users face. This information belongs in the logs.
Trivial: rename a field

We're about to introduce more monitors, so call this one
what it is: update_monitor.
Monitor network status

For now, we don't do anything. In the future, we want to hold
installs until we go online.
Handle offline installation sensibly

This commit adds a new app state, GS_APP_STATE_QUEUED. Apps
go in this state when you try to install them while offline.
This is done in the plugin loader, who puts such apps on its
list of pending apps. In the UI, queued apps show a 'Pending'
label and a cancel button. When network becomes available, the
plugin loader installs queued apps. The list of queued apps is
written out to ~/.local/share/gnome-software/queued-installs,
and reloaded from there when gnome-software is started.

https://bugzilla.gnome.org/show_bug.cgi?id=710114