Network config: Show error messages in button container

This CL:
* Moves the 'error' message display to the host dialogs so that it
  can be displayed in the button container which is always shown at
  the bottom of the dialog.
* Eliminates the 'Connecting' message which is no longer necessary
  since we now close the dialogs as soon as the asynchronous
  connection attempt starts.

TBR=stevenjb@chromium.org

(cherry picked from commit 0f54d8999aa322ffb5b58b103c1b24ea1529fecf)

Bug: 812477
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I5fa17ed3b568f689bd76c71b9a921010a81bf56e
Reviewed-on: https://chromium-review.googlesource.com/922795
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Kevin Cernekee <cernekee@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#538341}
Reviewed-on: https://chromium-review.googlesource.com/940430
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/branch-heads/3325@{#613}
Cr-Branched-From: bc084a8b5afa3744a74927344e304c02ae54189f-refs/heads/master@{#530369}
6 files changed