webadmin: Remove Storage connection on import domain failure

Remove storage server connection if we failed to import an existing Storage Domain.

Change-Id: I553a311fe8856b9cd5825405fef39e91e4c6e771
Bug-Url: https://bugzilla.redhat.com/1136930
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>