web: Close other dialogs before opening "Disconnected".

We can't have two dialogs open at the same time, and "Disconnected" is
opened asynchronously.

For convenience and robustness, we also close other dialogs before
opening the #error-popup although the code is usually careful to close
any open dialog before showing errors.

Closes #684
Fixes #667

Reviewed-by: Stef Walter <stefw@redhat.com>