app: Correct the logic of selecting URI

Seems the logic of selecting the argument to Wizard.open_with_uri() was
inverted.
wizard: Set UI state on App, not AppWindow

This fixes the issue of Boxes ending up in weird state if passed an
unknown (to Boxes) media through commandline (i-e when review is
skipped).
wizard: Don't try to create the box in two places

'Create' button should only set the appropriate wizard page as the
appropriate page setter already handles the box creation.
util-app: Don't abort on inability to read /proc files

Calling GLib.error() implies a call to abort() so its better if we
display the error with GLib.critical().

https://bugzilla.gnome.org/show_bug.cgi?id=729345
util-app: Improved critical messages

Ensure path of the file, that we failed to read is included in the
messages.

https://bugzilla.gnome.org/show_bug.cgi?id=729345
util-app: Turn 2 criticals into warnings

These messages are not severe enough to be criticals.

https://bugzilla.gnome.org/show_bug.cgi?id=729345
Release 3.12.3