webadmin: Aggregate errors when importing networks

When importing several networks, some imports may fail while others
succeed; all failures should be aggregated into one dialog to be
surveyed by the user.

Change-Id: I5230847a055a8a58f4596290e9ee5d2af2b09784
Bug-Url: https://bugzilla.redhat.com/1048752
Signed-off-by: Lior Vernia <lvernia@redhat.com>