webadmin: move dialog - modified warnings and styling

In order to provide a better indication to which of the
selected disks cannot be moved/copied:

* Added a list of problematic disks [1] into the dialog's warning.
* Added an alert icon next to the warning.
* Altered warnings position and style.

[1] Disks that cannot be moved/copied (e.g. since the source/target
    domain is not available).

Change-Id: Ib327915336dfbbdd6e71665b5d3ca603a8843698
Bug-Url: https://bugzilla.redhat.com/975096
Signed-off-by: Daniel Erez <derez@redhat.com>