[CrOS Network] Fix configuration error UI.

This CL removes individual error() and last_error() accessors from
NetworkState and exposes a GetError() function instead, which returns
whatever is the most recent error.

This CL also clears this error when a network has been set up to ensure
that networks are still connectable if they've just been configured with
a new password.

This CL fixes a regression which would show network *details* rather
than *configuration* when clicking a connection error notification.

Bug: 974737
Change-Id: I8528f848a441a1ae748e2c79baea4583d48bef6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672015
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672283}
10 files changed